virtio-devices: Add support for adding a single memory region

Assuming vhost-user devices support CONFIGURE_MEM_SLOTS protocol
feature, we introduce a new method to the VirtioDevice trait in order to
update one single memory at a time.

In case CONFIGURE_MEM_SLOTS is not supported by the backend (feature not
acked), we fallback onto the current way of updating the memory
mappings, that is with SET_MEM_TABLE.

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

virtio-devices/src/device.rs

@@ -16,7 +16,7 @@ use std::sync::{
     Arc, Barrier,
 };
 use std::thread;
-use vm_memory::{GuestAddress, GuestMemoryAtomic, GuestMemoryMmap, GuestUsize};
+use vm_memory::{GuestAddress, GuestMemoryAtomic, GuestMemoryMmap, GuestRegionMmap, GuestUsize};
 use vm_migration::{MigratableError, Pausable};
 use vm_virtio::VirtioDeviceType;
 use vmm_sys_util::eventfd::EventFd;
@@ -143,6 +143,13 @@ pub trait VirtioDevice: Send {
         Ok(())
     }
 
+    fn add_memory_region(
+        &mut self,
+        _region: &Arc<GuestRegionMmap>,
+    ) -> std::result::Result<(), Error> {
+        Ok(())
+    }
+
     /// Returns the list of userspace mappings associated with this device.
     fn userspace_mappings(&self) -> Vec<UserspaceMapping> {
         Vec::new()