vpc: Return 0 from vpc_co_create() on success

vpc: Return 0 from vpc_co_create() on success

blockdev_create_run() directly uses .bdrv_co_create()'s return value as
the job's return value.  Jobs must return 0 on success, not just any
nonnegative value.  Therefore, using blockdev-create for VPC images may
currently fail as the vpc driver may return a positive integer.

Because there is no point in returning a positive integer anywhere in
the block layer (all non-negative integers are generally treated as
complete success), we probably do not want to add more such cases.
Therefore, fix this problem by making the vpc driver always return 0 in
case of success.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>