build: Expose and use "sev_snp" feature on virtio-devices

Code in this crate is conditional on this feature so it necessary to
expose as a new feature and use that feature as a dependency when the
feature is enabled on the vmm crate.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

virtio-devices/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 
 [features]
 default = []
+sev_snp = []
 
 [dependencies]
 anyhow = "1.0.81"