qemu.tests.multi_disk_random_hotplug: Improve the unplug part

This patch changes the handling of unplug. The simplu_unplug has sometimes
problem detecting correctly the unplug status, as it might take seconds
before the disk is removed (monitor returns output, but qemu still
waits for OS repsonse before it removes the device from qtree).

This custom handler waits up to 1s for device to be removed and even
when it's not proved to be removed it removes it from representation.
We can afford this as all devices are again checked later in the test
and the VM state is for the whole period set to dirty.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk_random_hotplug: Improve logging

Add context for easier debugging.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk_random_hotplug: Put images_define in try/catch

The qdev.images_define_by_variables needs to be in the try/except
block in order to work as expected (remove format and try another round)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk_random_hotplug: raise exception if no disks added

With incorrect configuration it might happen that no disks can be added.
Throw exception instead of simple warning and clean test run.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #1336 from ldoktor/add_device

qemu.tests.multi_disk_random_hotplug: Couple of little improvements and bugfixes