qemu-iotests: Avoid unnecessary sleeps

Test cases 030, 041 and 055 used to sleep for a second after calling
block-job-pause to make sure that the block job had time to actually
get into paused state. We can instead poll its status and use that one
second only as a timeout.

The tests also slept a second for checking that the block jobs don't
make progress while being paused. Half a second is more than enough for
this.

These changes reduce the total time for the three tests by 25 seconds on
my laptop (from 155 seconds to 130).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
