iotests: 153: Wait for an answer to QMP commands

There are various actions in this test that must be executed
sequentially, as the result of it depends on the state triggered by the
previous one.

If the last argument of _send_qemu_cmd() is an empty string, it just
sends the QMP commands without waiting for an answer. While unlikely, it
may happen that the next action in the test gets invoked before QEMU
processes the QMP request.

This issue seems to be easier to reproduce on servers with limited
resources or highly loaded.

With this change, we wait for an answer on all _send_qemu_cmd() calls.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>