tools/pygrub: store kernels in /var/run/xen/pygrub

tools/pygrub: store kernels in /var/run/xen/pygrub

Move location of temporary bootfiles from /var/run/xend/boot to
/var/run/xen/pygrub. Create the subdirectory if does not exist.
The <dir> argument --output-directory must be an existing directory.

The reason for this change is that all entrys below /var/run have to be
created at runtime in case /var/run is cleared on every boot.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>