physmem: Introduce ram_block_discard_guest_memfd_range()

physmem: Introduce ram_block_discard_guest_memfd_range()

When memory page is converted from private to shared, the original
private memory is back'ed by guest_memfd. Introduce
ram_block_discard_guest_memfd_range() for discarding memory in
guest_memfd.

Based on a patch by Isaku Yamahata <isaku.yamahata@intel.com>.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240320083945.991426-12-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>