block/vvfat: Plug memory leak in enable_write_target()

I figure the leak originated in bdrv_create2(), and was duplicated
into callers when commit 91a073a dropped that function.  Looks like
the other places have since been fixed.

Spotted by Coverity.

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