KVM: x86: introduce is_pae_paging

Checking for 32-bit PAE is quite common around code that fiddles with
the PDPTRs.  Add a function to compress all checks into a single
invocation.

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