qcow2: Always execute preallocate() in a coroutine

qcow2: Always execute preallocate() in a coroutine

Some qcow2 functions (at least perform_cow()) expect s->lock to be
taken.  Therefore, if we want to make use of them, we should execute
preallocate() (as "preallocate_co") in a coroutine so that we can use
the qemu_co_mutex_* functions.

Signed-off-by: Max Reitz <[email protected]>
Message-id: 20171009215533[email protected]
Cc: [email protected]
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Max Reitz <[email protected]>