tools/hotplug: fix systemd service files for dom0

ConditionVirtualization= checks if a guest is active. It is not supposed
to detect host capabilities. The current implementation happens to work
because systemd-detect-virt from v208 returns also 'xen' in a dom0. In
v210 and later 'none' is returned and all service files are not started.

Adjust the checks to detect a dom0. Mounting xenfs depends on /proc/xen,
but should only be done for pvops because xenfs exists only there.  All
other service files should not be started in domU. The file
/proc/xen/capabilities exists in both dom0 and domU,
ConditionVirtualization=!xen is true in a dom0.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>