libxl: Handle Linux stubdomain specific QEMU options.

libxl: Handle Linux stubdomain specific QEMU options.

This patch creates an appropriate command line for the QEMU instance
running in a Linux-based stubdomain.

NOTE: a number of items are not currently implemented for Linux-based
stubdomains, such as:
- save/restore
- QMP socket
- graphics output (e.g., VNC)

Signed-off-by: Eric Shelton <eshelton@pobox.com>
Simon:
 * fix disk path
 * fix cdrom path and "format"

Signed-off-by: Simon Gaiser <simon@invisiblethingslab.com>
[drop Qubes-specific parts]
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Allow setting stubdomain_ramdisk independently from stubdomain_kernel
Add a qemu- prefix for qemu-stubdom-linux-{kernel,rootfs} since stubdom
doesn't convey device-model.  Use qemu- since this code is qemu specific.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>