blockdev: Plug memory leak in drive_init()

bs_opts is leaked on all paths from its qdev_new() that don't got
through blockdev_init().  Add the missing QDECREF(), and zap bs_opts
after blockdev_init(), so the new QDECREF() does nothing when we go
through blockdev_init().

Leak introduced in commit f298d07.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
