Detected by Coverity.  Leak introduced in commit 8866eed0.

Two bugs here:
1. logfd wasn't closed on all return paths
2. if we failed to mark a domain autodestroy, then the domain
was not made transient but we still returned success
Signed-off-by: NAlex Jia <ajia@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>