KVM: SVM: Introduce ops for the post gfn map and unmap

When SEV-SNP is enabled in the guest VM, the guest memory pages can
either be a private or shared. A write from the hypervisor goes through
the RMP checks. If hardware sees that hypervisor is attempting to write
to a guest private page, then it triggers an RMP violation #PF.

To avoid the RMP violation with GHCB pages, added new post_{map,unmap}_gfn
functions to verify if its safe to map GHCB pages.  Uses a spinlock to
protect against the page state change for existing mapped pages.

Need to add generic post_{map,unmap}_gfn() ops that can be used to verify
that its safe to map a given guest page in the hypervisor.

This patch will need to be revisited later after consensus is reached on
how to manage guest private memory as probably UPM private memslots will
be able to handle this page state change more gracefully.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off by: Ashish Kalra <ashish.kalra@amd.com>

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