vmm: Add epoll_ctl() syscall to vCPU seccomp filter

Fix seccomp violation when trying to add the out FD to the epoll loop
when the serial buffer needs to be flushed.

0x00007ffff7dc093e in epoll_ctl () at ../sysdeps/unix/syscall-template.S:120
0x0000555555db9b6d in epoll::ctl (epfd=56, op=epoll::ControlOptions::EPOLL_CTL_MOD, fd=55, event=...)
    at /home/rob/.cargo/registry/src/github.com-1ecc6299db9ec823/epoll-4.3.1/src/lib.rs:155
0x00005555556f5127 in vmm::serial_buffer::SerialBuffer::add_out_poll (self=0x7fffe800b5d0) at vmm/src/serial_buffer.rs:101
0x00005555556f583d in vmm::serial_buffer::{impl#1}::write (self=0x7fffe800b5d0, buf=...) at vmm/src/serial_buffer.rs:139
0x0000555555a30b10 in std::io::Write::write_all<vmm::serial_buffer::SerialBuffer> (self=0x7fffe800b5d0, buf=...)
    at /rustc/59eed8a2aac0230a8b53e89d4e99d55912ba6b35/library/std/src/io/mod.rs:1527
0x0000555555ab82fb in devices::legacy::serial::Serial::handle_write (self=0x7fffe800b520, offset=0, v=13) at devices/src/legacy/serial.rs:217
0x0000555555ab897f in devices::legacy::serial::{impl#2}::write (self=0x7fffe800b520, _base=1016, offset=0, data=...) at devices/src/legacy/serial.rs:295
0x0000555555f30e95 in vm_device:🚌:Bus::write (self=0x7fffe8006ce0, addr=1016, data=...) at vm-device/src/bus.rs:235
0x00005555559406d4 in vmm::vm::{impl#4}::pio_write (self=0x7fffe8009640, port=1016, data=...) at vmm/src/vm.rs:459

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/seccomp_filters.rs

@@ -588,6 +588,7 @@ fn vcpu_thread_rules() -> Result<Vec<(i64, Vec<SeccompRule>)>, BackendError> {
         (libc::SYS_close, vec![]),
         (libc::SYS_dup, vec![]),
         (libc::SYS_exit, vec![]),
+        (libc::SYS_epoll_ctl, vec![]),
         (libc::SYS_fstat, vec![]),
         (libc::SYS_futex, vec![]),
         (libc::SYS_getrandom, vec![]),