memory-device: forward errors in get_region_size()/get_plugged_size()

memory-device: forward errors in get_region_size()/get_plugged_size()

Let's properly forward the errors, so errors from get_region_size() /
get_plugged_size() can be handled.

Users right now call both functions after the device has been realized,
which is will never fail, so it is fine to continue using error_abort.

While at it, remove a leftover error check (suggested by Igor).

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20181005092024.14344-8-david@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>