KVM: MMU: fully check zero bits for sptes

The #PF with PFEC.RSV = 1 is designed to speed MMIO emulation, however,
it is possible that the RSV #PF is caused by real BUG by mis-configure
shadow page table entries

This patch enables full check for the zero bits on shadow page table
entries (which includes not only bits reserved by the hardware, but also
bits that will never be set in the SPTE), then dump the shadow page table
hierarchy.

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>