iotests/028: Fix for long $TEST_DIRs

iotests/028: Fix for long $TEST_DIRs

For long test image paths, the order of the "Formatting" line and the
"(qemu)" prompt after a drive_backup HMP command may be reversed.  In
fact, the interaction between the prompt and the line may lead to the
"Formatting" to being greppable at all after "read"-ing it (if the
prompt injects an IFS character into the "Formatting" string).

So just wait until we get a prompt.  At that point, the block job must
have been started, so "info block-jobs" will only return "No active
jobs" once it is done.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>