Since softmmu & AMD nested shadow page tables and guest page tables have
the same format, split reset_rsvds_bits_mask so that the logic can be
reused by later patches which check zero bits on sptes
Signed-off-by: NXiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>