qcow: fix a reference leak

Since 42a3e1ab qemu asserts when using the
vvfat driver:

git clone git://qemu.org/qemu.git


cd qemu
./configure --target-list=ppc-softmmu --enable-debug
make -j8
mkdir foo
touch foo/hello
./ppc-softmmu/qemu-system-ppc -M prep --nographic --monitor null             \
                              -hda fat:rw:./foo

"Ctrl-C"

qemu-system-ppc: block.c:3368: bdrv_close_all: Assertion                     \
   `((&all_bdrv_states)->tqh_first == ((void *)0))' failed.

This is because we reference bs twice in qcow_co_create(..) one time in
bdrv_open_blockdev_ref(..) and in blk_insert_bs(..) but we unref it only once
in blk_unref which leads to the reference leak.

Note that I didn't tested much QCOW after this change as I don't use it much.

Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>