Add vhost-user helper to get MemoryRegion data

Add vhost-user helper to get MemoryRegion data

When setting the memory tables, qemu uses a memory region's userspace
address to look up the region's MemoryRegion struct. Among other things,
the MemoryRegion contains the region's offset and associated file
descriptor, all of which need to be sent to the backend.

With VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS, this logic will be
needed in multiple places, so before feature support is added it
should be moved to a helper function.

This helper is also used to simplify the vhost_user_can_merge()
function.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <1588533678-23450-3-git-send-email-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>