KVM: MMU: check reserved bits only if CR4.PSE=1 or CR4.PAE=1

- Check reserved bits only if CR4.PAE=1 or CR4.PSE=1 when guest #PF occurs
- Fix a typo in reset_rsvds_bits_mask()

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>