To avoid the collision for creating USB controllers in machine->init()
and -device xx command line, it needs to set usb=off to avoid one USB
controller created in machine->init(). So that libvirt can use -device
or -usb to create USB controller sucessfully.
So QEMU_CAPS_MACHINE_USB_OPT capability is added, and it is for QEMU
v1.3.0 onwards which supports USB option.
Signed-off-by: NLi Zhang <zhlcindy@linux.vnet.ibm.com>