tests.mac_change: Support file transfer test after mac changing

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.cfg.device_assignment: Add new device_assignment sub case configure.

Signed-off-by: Feng Yang <fyang@redhat.com>
share.cfg.guest-os: Limit pf_mac_change test case on guest.

pf_mac_change case could not work on WinXP.
Set change_cmd parameter for Windows/Linux guest.

Signed-off-by: Feng Yang <fyang@redhat.com>
qemu.tests.pci_hotunplug: Support sub test before/after unplugging.

Signed-off-by: Feng Yang <fyang@redhat.com>