xen_xs: name xendConfigVersion magic numbers

xen_xs: name xendConfigVersion magic numbers

libvirt supports 4 different versions of the user-land XenD daemon. When
queried the daemon just returns its generation number, which is hard to
match to the version of the Xen tools.

Replace the magic generation numbers by named enum definitions to
improve code readability.

Signed-off-by: Philipp Hahn <hahn@univention.de>