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>
qemu.tests: Add macvtap_mac_change test case.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #110 from ypu/pci_hotplug

qemu.tests: Add some subtests for pci_hotplug
Merge pull request #122 from FengYang/macvtap_mac_change

Macvtap mac change
generic.tests: Add reboot step for mac_change and two sub cases

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