qemu.tests.cfg: Add test for 8 devices on usb-hub

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.test.usb_hotplug: Get usb device type from its own params

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Add missing equal sign

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>