There are at least two different errors that can occur in
block_job_set_speed(): the job might not support setting speeds or the
value might be invalid.

Use the Error mechanism to report the error where it occurs.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>