qcow2: set bdi->is_dirty

qcow2: set bdi->is_dirty

Set bdi->is_dirty, so that qemu-img info could show dirty flag.

After this commit the following check will show '"dirty-flag": true':

./build/qemu-img create -f qcow2 -o lazy_refcounts=on x 1M
./build/qemu-io x
qemu-io> write 0 1M

 After "write" command success, kill the qemu-io process:

kill -9 <qemu-io pid>

./build/qemu-img info --output=json x

This will show '"dirty-flag": true' among other things. (before this
commit it shows '"dirty-flag": false')

Note, that qcow2's dirty-bit is not a "dirty bit for the image". It
only protects qcow2 lazy refcounts feature. So, there are a lot of
conditions when qcow2 session may be not closed correctly, but bit is
0. Still, when bit is set, the last session is definitely not finished
correctly and it's better to report it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210504160656.462836-1-vsementsov@virtuozzo.com>
Tested-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>