vm-virtio: Get userspace mappings from VirtioDevice

Introduce new getter function to the VirtioDevice trait, as it will
allow the caller to retrieve the list of userspace mappings associated
with the device.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-virtio/src/device.rs

@@ -34,6 +34,15 @@ pub trait VirtioInterrupt: Send + Sync {
 pub type VirtioIommuRemapping =
     Box<dyn Fn(u64) -> std::result::Result<u64, std::io::Error> + Send + Sync>;
 
+#[derive(Clone)]
+pub struct UserspaceMapping {
+    pub host_addr: u64,
+    pub mem_slot: u32,
+    pub addr: GuestAddress,
+    pub len: GuestUsize,
+    pub mergeable: bool,
+}
+
 #[derive(Clone)]
 pub struct VirtioSharedMemory {
     pub offset: u64,
@@ -120,6 +129,11 @@ pub trait VirtioDevice: Send {
     fn update_memory(&mut self, _mem: &GuestMemoryMmap) -> std::result::Result<(), Error> {
         Ok(())
     }
+
+    /// Returns the list of userspace mappings associated with this device.
+    fn userspace_mappings(&self) -> Vec<UserspaceMapping> {
+        Vec::new()
+    }
 }
 
 /// Trait providing address translation the same way a physical DMA remapping