Allow configuration of QEMU parallel port emulation

By default the toolstack does not instruct QEMU at all about parallel
port emulation. This causes QEMU to emulate a parallel port by default.
This commit allows for a "parallel" key to be specified in the platform
VM record parameter. The string used as this key will be passed to QEMU
as "-parallel <string>". For example, one can disable the parallel
port emulation completely by issuing:
xe vm-param-set uuid=<vm_sel> platform:parallel=none
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
Merge pull request #1022 from jonludlam/CP-4147

Cp 4147 - CLEARWATER