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-uuid> platform:parallel=none

Signed-off-by: John Else <john.else@citrix.com>