IGVM no longer implies the MSHV backend at the feature layer. Gate the
loader paths that use MSHV page types, MSHV SNP launch defaults, or
MSHV-specific CPUID page rewriting on the MSHV feature.
This preserves existing MSHV behavior while letting KVM SEV-SNP use the
shared IGVM loader without compiling the full MSHV backend.
Assisted-by: Copilot:GPT-5.5
Signed-off-by: Wei Liu <liuwe@microsoft.com>