hypervisor: Keep GHCB mapped address for each VCPU on MSHV

For confidential VM on MSHV, GHCB page is the communication
method between guest and host. All the CVM exits, VMM
needs to read and write to the GHCB page. MSHV provides
an option to remap the page to the root partition. This
way VMM could directly read and write to the page and
skip extra IOCTL and hypercall. This improvement makes
the IO 10% faster.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

hypervisor/src/vm.rs

@@ -251,6 +251,12 @@ pub enum HypervisorVmError {
     #[cfg(feature = "sev_snp")]
     #[error("Failed to modify GPA host access: {0}")]
     ModifyGpaHostAccess(#[source] anyhow::Error),
+    ///
+    /// Failed to mmap
+    ///
+    #[cfg(feature = "sev_snp")]
+    #[error("Failed to mmap:")]
+    MmapToRoot,
 }
 ///
 /// Result type for returning from a function