blockdev: Plug memory leak in blockdev_init()

blockdev_init() leaks bs_opts when qemu_opts_create() fails, i.e. when
the ID is bad.  Missed in commit ec9c10d.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
