KVM: Portability: Split mmu-related static inline functions to mmu.h

Since these functions need to know the details of kvm or kvm_vcpu structure,
it can't be put in x86.h.  Create mmu.h to hold them.

Signed-off-by: Zhang Xiantao <xiantao.zhang@intel.com>
Acked-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>