KVM: MMU: retain more available bits on mmio spte

Let mmio spte only use bit62 and bit63 on upper 32 bits, then bit 52 ~ bit 61
can be used for other purposes

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>