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/vhost_user/mod.rs

@@ -84,9 +84,13 @@ pub enum Error {
     MasterReqHandlerCreation(vhost::vhost_user::Error),
     /// Set slave request fd failed.
     VhostUserSetSlaveRequestFd(vhost::Error),
+    /// Add memory region failed.
+    VhostUserAddMemReg(VhostError),
     /// Invalid used address.
     UsedAddress,
     /// Invalid features provided from vhost-user backend
     InvalidFeatures,
+    /// Missing file descriptor for the region.
+    MissingRegionFd,
 }
 type Result<T> = std::result::Result<T, Error>;