pygrub: introduce easier to parse output format

pygrub: introduce easier to parse output format

libxl would rather like to parse the output of pygrub. Rather than
implement an SXP parser in libxl add a --output-format option to
pygrub which can select an alternative, simpler to parse,
format. Available formats are:
    sxp:	current SXP output format;
    simple:	simple key+value output with \n separating item ( for
		debugging). key and value are separated by a single
		space (and key therefore cannot contain a space);
    simple0:	as simple but with \0 as a separator;

Also add --output-directory to allow temporary files to be placed
somewhere other than /var/run/xend/boot.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>