qemu.tests.multi_disk_random_hotplug: Improve stop_stresser function

Stop stresser should destroy the VM ungracefully in case login fails.
Also check the vm.verify_alive before using monitor on "hot" places.

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

Log the hotplug device's name for easier log analyzing.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.multi_disk_random_hotplug: Check for I/O Errors

Check for I/O Errors before the test end and raise Warning in such case.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>