qcow2: make qcow2_co_create2() a coroutine_fn

qcow2: make qcow2_co_create2() a coroutine_fn

qcow2_create2() calls qemu_co_mutex_lock().  Only a coroutine_fn may
call another coroutine_fn.  In fact, qcow2_create2 is always called from
coroutine context.

Rename the function to add the "co" moniker and add coroutine_fn.

Reported-by: Marc-André Lureau <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20170705102231[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>