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
Hi ypu,
Seems there are two method to unplug disk
1. hotplug - > unplug
2. attach device to cmd -> unplug
seems you are using the method one.