KVM: x86/mmu: Move kvm_mmu_available_pages() into mmu.c

Move kvm_mmu_available_pages() from mmu.h to mmu.c, it has a single
caller and has no business being exposed via mmu.h.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200622202034.15093-3-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>