xl: add "device_model_args" to pass arbitrary extra arguments to device model

xl: add "device_model_args" to pass arbitrary extra arguments to device model

The libxl support was already in place so simply plumb it through.
This allows for passing debug options to the device model and provides
a method to work around missing toolstack functionality.

e.g. xl does not current support floppy disks but adding:
     device_model_args = [ "-fda", "/scratch/fdboot.img" ]
allowed me to boot FreeDOS from a floppy image.

I was unable to find any equivalent functionality in xend so this is a
new xl feature.

Moved xmalloc/xrealloc earlier to allow use from parse_config_data.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>