KVM: improve hva_to_pfn() readability

Improve vma handling code readability in hva_to_pfn() and fix
async pf handling code to properly check vma returned by find_vma().

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>