virt: use XENFEAT_dom0 to detect the hardware domain (#6442, #6662) (#7581)

virt: use XENFEAT_dom0 to detect the hardware domain (#6442, #6662) (#7581)

The detection of ConditionVirtualisation= relies on the presence of
/proc/xen/capabilities. If the file exists and contains the string
"control_d", the running system is a dom0 and VIRTUALIZATION_NONE should
be set. In case /proc/xen exists, or some sysfs files indicate "xen",
VIRTUALIZATION_XEN should be set to indicate the system is a domU.

With an (old) xenlinux based kernel, /proc/xen/capabilities is always
available and the detection described above works always. But with a
pvops based kernel, xenfs must be mounted on /proc/xen to get
"capabilities". This is done by a proc-xen.mount unit, which is part of
xen.git. Since the mounting happens "late", other units may be scheduled
before "proc-xen.mount". If these other units make use of
"ConditionVirtualisation=", the virtualization detection returns
incorect results. detect_vm() will set VIRTUALIZATION_XEN because "xen"
is found in sysfs. This value will be cached. Once xenfs is mounted, the
next process that runs detect_vm() will get VIRTUALIZATION_NONE.

This misdetection can be fixed by using
/sys/hypervisor/properties/features, which exports the value returned by
the "XENVER_get_features" hypercall. If the bit XENFEAT_dom0 is set, the
domain is the "hardware domain". It is supposed to have permissions to
access all hardware. The used sysfs file is available since v2.6.31.

The commonly used term "dom0" refers to the control domain which runs
the toolstack and has access to all hardware. But the virtualization
host may be configured such that one dedicated domain becomes the
"hardware domain", and another one the "toolstack domain".