qemu.test: add reboot step for pci_hotplug testing

We need step reboot guest after hotplug/unplug pci device to verify guest
not crash or hang when reboot;

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #842 from xutian/add_reboot_step

qemu.test: add reboot step for pci_hotplug testing