vmm: Add 'shutdown()' to vCPU seccomp filter

This is required when hot-removing a vfio-user device. Details code path
below:

Thread 6 "vcpu0" received signal SIGSYS, Bad system call.
[Switching to Thread 0x7f8196889700 (LWP 2358305)]
0x00007f8196dae7ab in shutdown () at ../sysdeps/unix/syscall-template.S:78
78	T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
(gdb) bt
  0x00007f8196dae7ab in shutdown () at ../sysdeps/unix/syscall-template.S:78
  0x000056189240737d in std::sys::unix::net::Socket::shutdown ()
    at library/std/src/sys/unix/net.rs:383
  std::os::unix::net::stream::UnixStream::shutdown () at library/std/src/os/unix/net/stream.rs:479
  0x000056189210e23d in vfio_user::Client::shutdown (self=0x7f8190014300)
    at vfio_user/src/lib.rs:787
  0x00005618920b9d02 in <pci::vfio_user::VfioUserPciDevice as core::ops::drop::Drop>::drop (
    self=0x7f819002d7c0) at pci/src/vfio_user.rs:551
  0x00005618920b8787 in core::ptr::drop_in_place<pci::vfio_user::VfioUserPciDevice> ()
    at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ptr/mod.rs:188
  0x00005618920b92e3 in core::ptr::drop_in_place<core::cell::UnsafeCell<dyn pci::device::PciDevice>>
    () at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ptr/mod.rs:188
  0x00005618920b9362 in core::ptr::drop_in_place<std::sync::mutex::Mutex<dyn pci::device::PciDevice>> () at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ptr/mod.rs:188
  0x00005618920d8a3e in alloc::sync::Arc<T>::drop_slow (self=0x7f81968852b8)
    at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/alloc/src/sync.rs:1092
  0x00005618920ba273 in <alloc::sync::Arc<T> as core::ops::drop::Drop>::drop (self=0x7f81968852b8)
    at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/alloc/src/sync.rs:1688
 0x00005618920b76fb in core::ptr::drop_in_place<alloc::sync::Arc<std::sync::mutex::Mutex<dyn pci::device::PciDevice>>> ()
    at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/ptr/mod.rs:188
 0x0000561891b5e47d in vmm::device_manager::DeviceManager::eject_device (self=0x7f8190009600,
    pci_segment_id=0, device_id=3) at vmm/src/device_manager.rs:4000
 0x0000561891b674bc in <vmm::device_manager::DeviceManager as vm_device:🚌:BusDevice>::write (
    self=0x7f8190009600, base=70368744108032, offset=8, data=&[u8](size=4) = {...})
    at vmm/src/device_manager.rs:4625
 0x00005618921927d5 in vm_device:🚌:Bus::write (self=0x7f8190006e00, addr=70368744108040,
    data=&[u8](size=4) = {...}) at vm-device/src/bus.rs:235
 0x0000561891b72e10 in <vmm::vm::VmOps as hypervisor::vm::VmmOps>::mmio_write (
    self=0x7f81900097b0, gpa=70368744108040, data=&[u8](size=4) = {...}) at vmm/src/vm.rs:378
 0x0000561892133ae2 in <hypervisor::kvm::KvmVcpu as hypervisor::cpu::Vcpu>::run (
    self=0x7f8190013c90) at hypervisor/src/kvm/mod.rs:1114
 0x0000561891914e85 in vmm::cpu::Vcpu::run (self=0x7f819001b230) at vmm/src/cpu.rs:348
 0x000056189189f2cb in vmm::cpu::CpuManager::start_vcpu::{{closure}}::{{closure}} ()
    at vmm/src/cpu.rs:953

Signed-off-by: Bo Chen <chen.bo@intel.com>

vmm/src/seccomp_filters.rs

@@ -686,6 +686,7 @@ fn vcpu_thread_rules() -> Result<Vec<(i64, Vec<SeccompRule>)>, BackendError> {
         (libc::SYS_rt_sigprocmask, vec![]),
         (libc::SYS_rt_sigreturn, vec![]),
         (libc::SYS_sendmsg, vec![]),
+        (libc::SYS_shutdown, vec![]),
         (libc::SYS_sigaltstack, vec![]),
         (libc::SYS_tgkill, vec![]),
         (libc::SYS_tkill, vec![]),