qemu-iotests: add block-stream speed value test case

Add tests to exercise the InvalidParameter 'speed' error code path, as
well as the regular success case for setting the speed.  The
block-stream 'speed' parameter allows the speed limit of the job to be
applied immediately when the job starts instead of issuing a separate
block-job-set-speed command later.  If the parameter has an invalid
value we expect to get an error and the job is not created.

It turns out that cancelling a block job is a common operation in these
test cases, let's extract a cancel_and_wait() function instead of
duplicating the QMP commands.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
