qemu.tests.multi_disk_random_hotplug: Parallel variant of this test

Hi guys,

this pull request adds parallel variant of this disk hotplug test. It also contains couple of qemu_devices bugfixes, which were found during test development.

Please note that `virtio_scsi` disks in parallel mode usually destroy the VM, which is already reported. Also `virtio_scsi` and `lsi_scsi` sometimes fail to initialize in guest, which is also a known issue.

Kind regards,
Lukáš