vhost: return task creation error instead of NULL

Lets callers distinguish why the vhost task creation failed. No one
currently cares why it failed, so no real runtime change from this
patch, but that will not be the case for long.

Signed-off-by: Keith Busch <[email protected]>
Message-ID: <[email protected]>
Reviewed-by: Mike Christie <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
3 files changed