qemu.tests: Add some subtests for pci_hotplug

Subtests add to pci_hotplug:
  - guest_s4 support for block device
  - pci_hotplug with system_reset
  - pci_hotplug with stop continue
  - pci_hotplug with block resize
Thank you for youe help, 
find the reason which confuse me as no judging when virtio-blk and virtio-scsi device.
there is addition step to hotplug virtio-scsi controller.

Some comments:
1.  line 122  error.context("Adding SCSI controller.") --> seems need to add logging.info 
2.  line 144   error.context("Adding driver.") --> same as above
3.  line 152  error.context("Adding pci device with command 'device_add'") --> same as above

4.  seems we only check if the controller is hotplugged with lspci, didn't check if the device is shown in guest, correct me if something mis-unstanding.

Regards,
Suqin