qemu-img: eliminate memory leak

Not particularly important since qemu-img exits immediately after
calling img_rebase, but easily fixed.  Coverity says thanks.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
