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

Also add virtio_scsi block hotplug support and fix a cmd problem for
hmp and qmp monitor.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>