vfio_user: Improve flags for DMA_MAP command

Replace the use of an enum with a bitfield representation which means
that is now possible to logical OR flags together.

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

vfio_user/Cargo.toml

@@ -5,6 +5,7 @@ authors = ["The Cloud Hypervisor Authors"]
 edition = "2021"
 
 [dependencies]
+bitflags = "1.3.2"
 libc = "0.2.139"
 log = "0.4.17"
 serde = { version = "1.0.151", features = ["rc"] }