x86/HVM: restrict HVMOP_set_mem_type

x86/HVM: restrict HVMOP_set_mem_type

Permitting arbitrary type changes here has the potential of creating
present P2M (and hence EPT/NPT/IOMMU) entries pointing to an invalid
MFN (INVALID_MFN truncated to the respective hardware structure field's
width). This would become a problem the latest when something real sat
at the end of the physical address space; I'm suspecting though that
other things might break with such bogus entries.

Along with that drop a bogus (and otherwise becoming stale) log
message.

Afaict the similar operation in p2m_set_mem_access() is safe.

This is XSA-92.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>