virInitialize() → xenRegister() → xenhypervisorInit() determines the
version of the Hypervisor. This breaks xencapstest when building as root
on a dom0 system, since xenHypervisorBuildCapabilities() adds the "hap"
and "viridian" features based on the detected version.

Add an optional parameter to xenhypervisorInit() to disable automatic
detection of the Hypervisor version. The passed in arguments are used
instead.
Signed-off-by: NPhilipp Hahn <hahn@univention.de>