ivshmem: Fix harmless misuse of Error

We reuse errp after passing it host_memory_backend_get_memory().  If
both host_memory_backend_get_memory() and the reuse set an error, the
reuse will fail the assertion in error_setv().  Fortunately,
host_memory_backend_get_memory() can't fail.

Pass it &error_abort to make our assumption explicit, and to get the
assertion failure in the right place should it become invalid.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-17-git-send-email-armbru@redhat.com>
