qemu.tests: nic_hotplug: Add a regression test

Add regress test for bug: Fix phys memory client for vhost

Stop/continue guest after hot-adding a nic, guest will
be panic, or guest network will be down.

Signed-off-by: Amos Kong <akong@redhat.com>
Acked-by:  Golita Yue <gyue@redhat.com>
qemu.tests: nic_hotplug: Update error context

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>