libvhost-user: Factor out search for memory region by GPA and simplify

libvhost-user: Factor out search for memory region by GPA and simplify

Memory regions cannot overlap, and if we ever hit that case something
would be really flawed.

For example, when vhost code in QEMU decides to increase the size of memory
regions to cover full huge pages, it makes sure to never create overlaps,
and if there would be overlaps, it would bail out.

QEMU commits 48d7c9757749 ("vhost: Merge sections added to temporary
list"), c1ece84e7c93 ("vhost: Huge page align and merge") and
e7b94a84b6cb ("vhost: Allow adjoining regions") added and clarified that
handling and how overlaps are impossible.

Consequently, each GPA can belong to at most one memory region, and
everything else doesn't make sense. Let's factor out our search to prepare
for further changes.

Reviewed-by: Raphael Norwitz <raphael@enfabrica.net>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20240214151701.29906-10-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>