virttest.test_setup: Update test_setup.PciAssignable() class.

This patch update:
Support get VF from specified PF.
Do not restart host network during testing.
Update the command used to release VF devices to make it
works in rhel7 host.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1908 from FengYang/PciAssignable2

virttest.test_setup: Update test_setup.PciAssignable() class.