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
@suqinhuang for 1~3 thanks for your reminding. And will add the logging.info in this script. 

But for step 4, there is some check for block device which can make sure the devices are exist inside guest. You can find them in the cfg files.:
            reference_cmd = lspci
            find_pci_cmd = 'lspci'
            pci_test_cmd = "echo %s; yes | mke2fs `fdisk -l 2>&1 | awk '/\/dev\/[sv]d[a-z]* doesn/ {print $2}'` | sort | tail -1"