xl and xm differ a bit in how <os> configuration is represented.
E.g. xl config supports <os><nvram .../></os> via its "bios"
setting.

Move the xenParseOS and xenFormatOS functions from xen_common.c
and copy to xen_xl.c and xen_xm.c so they can be customized for
xm vs xl config.  An unfortunate fallout is reordering of entries
in the test config files.