hostmem: introduce host_memory_backend_mr_inited()

We were checking this against memory region size of host memory
backend's mr field to see whether the mr has been inited. This is
efficient but less elegant. Let's make a helper for it to avoid
confusions, along with some notes.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1489151370-15453-2-git-send-email-peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
