memory-device: Fix memory pre-plug error API violations

memory-device: Fix memory pre-plug error API violations

memory_device_get_free_addr() dereferences @errp when
memory_device_check_addable() fails.  That's wrong; see the big
comment in error.h.  Introduced in commit 1b6d6af21b "pc-dimm: factor
out capacity and slot checks into MemoryDevice".

No caller actually passes null.

Fix anyway: splice in a local Error *err, and error_propagate().

Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20191204093625.14836-11-armbru@redhat.com>