x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()

x86/HVM: honor p2m_ram_ro in hvm_map_guest_frame_rw()

... and its callers.

While all non-nested users are made fully honor the semantics of that
type, doing so in the nested case seemed insane (if doable at all,
considering VMCS shadowing), and hence there the respective operations
are simply made fail.

One case not (yet) taken care of is that of a page getting transitioned
to this type after a mapping got established.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>