Allow disabling USB emulation on QEMU

By default, the toolstack will instruct QEMU to emulate a USB hub and
a USB tablet device for HVM guests. It does that by passing the
following parameters to QEMU: "-usb -usbdevice tablet".

This patch allows the disabling of USB emulation via a configurable
key in the VM record. When set, the toolstack will not pass any USB
emulation instructions to QEMU, therefore disabling the emulation.
For example, one can set the key as follows:
xe vm-param-set uuid=<vm_sel> platform:nousb=true