vmm: Make sev_snp depend on igvm

Currently both kvm and mshv require an IGVM file to boot a SEV-SNP VM.
This is already configured in the top-level cloud-hypervisor
Cargo.toml where sev_snp depends on igvm.

Add a similar dependency in the vmm crate which helps simplify some of
the in-code cfg blocks by removing the ones that are within a sev_snp
cfg block.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>

vmm/Cargo.toml

@@ -35,7 +35,7 @@ pvmemcontrol = ["devices/pvmemcontrol"]
 sev_snp = [
   "arch/sev_snp",
   "hypervisor/sev_snp",
-  "igvm_defs",
+  "igvm",
   "virtio-devices/sev_snp",
 ]
 tdx = ["arch/tdx", "hypervisor/tdx"]