qemu.test: make nic_hotplug support hot plug/unplug multi nics

This patch make nic_hotplug test support hot plug/unplug multi
nics with different nic_model.

This patch also make nic_hotplug support random unplug a nic that
have been pluged.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
qemu.test: Add multi nics hotplug/hotunplug test

This patch add multi nics hotplug/hotunplug test

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>