xenpaging: add cmdline interface for pager

xenpaging: add cmdline interface for pager

Introduce a cmdline handling for the pager. This simplifies libxl support,
debug and mru_size are not passed via the environment anymore.
The new interface looks like this:

xenpaging [options] -f <pagefile> -d <domain_id>
options:
 -d <domid>     --domain=<domid>         numerical domain_id of guest. This option is required.
 -f <file>      --pagefile=<file>        pagefile to use. This option is required.
 -m <max_memkb> --max_memkb=<max_memkb>  maximum amount of memory to handle.
 -r <num>       --mru_size=<num>         number of paged-in pages to keep in memory.
 -d             --debug                  enable debug output.
 -h             --help                   this output.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>