KVM: x86/mmu: Add MMU-internal header

Add mmu/mmu_internal.h to hold declarations and definitions that need
to be shared between various mmu/ files, but should not be used by
anything outside of the MMU.

Begin populating mmu_internal.h with declarations of the helpers used by
page_track.c.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200622202034.15093-4-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>