virttest: Support get VF from specified sr-iov PF.

We can set device_name to PF's devname in host.

If PF not specified, will try to get VF from all PF in turn.

Signed-off-by: Feng Yang <fyang@redhat.com>
sr-iov: virttest.test_setup: Try to get more pxception when VF/Pf clean up command fail.

Also delete self.dev_unbind_drivers.

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.test_setup: Add devices parameter for get_pf_devs() and get_vf_devs()

Signed-off-by: Feng Yang <fyang@redhat.com>
virttest.test_setup: Check available before load/unload self.driver.

Sleep 3s after loading self.driver.

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