build: use vfio-ioctls that supports MSHV

Disable default features and propagate hypervisor selection where
necessary.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

vm-device/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 [features]
 default = []
 kvm = ["vfio-ioctls/kvm"]
+mshv = ["vfio-ioctls/mshv"]
 
 [dependencies]
 anyhow = "1.0.43"