Fill the bits between 51..number-of-physical-address-bits in the
MTRR_PHYSMASKn variable range mtrr masks so that they're consistent
in the migration stream irrespective of the physical address space
of the source VM in a migration.
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>