x86/mm: Validate memory when changing the C-bit

Add the needed functionality to change pages state from shared
to private and vice-versa using the Page State Change VMGEXIT as
documented in the GHCB spec.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-22-brijesh.singh@amd.com