virttest: Init old_redirs as a empty dict

Make the type of old_redirs and self.redirs the same. Otherwise
it will always failed in the check step and set up the self.devices
to None.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
virttest: Update nic command line in create when devices already exist

As we will skip the command update if vm object already have self.devices
in qemu_vm. We need manually update the fds and vhostfds for it when we
get them from host.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1674 from ypu/guest_s4_hotplug

Guest s4 hotplug