qcow: initialize coroutine mutex

commit 52b8eb60132b27ad53476490e9d7579003390cfa added a mutex,
but never initialized it.  This caused a segfault.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
