KVM: x86: Add is_executable_pte()

Extracted from commit d3e328f2cb01 "kvm: x86: mmu: Verify that
restored PTE has needed perms in fast page fault".

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>