block: Fix leak in bdrv_create_file_fallback()

@options is leaked by the first two return statements in this function.

Note that blk_new_open() takes the reference to @options even on
failure, so all we need to do to fix the leak is to move the QDict
allocation down to where we actually need it.

Reported-by: Coverity (CID 1419884)
Fixes: fd17146c
       ("block: Generic file creation fallback")
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200225155618.133412-1-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>