memory: Introduce address_space_lookup_region

This introduces a wrapper for phys_page_find (before we complicate
address_space_translate with IOMMU translation).  This function will
also encapsulate locking and reference counting when we introduce
BQL-free dispatching.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>