x86/vPMU: document as unsupported

This is XSA-163.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
Note that the referenced link will only become active after public
disclosure.

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1482,8 +1482,9 @@ feature is switched on on Intel processo
 Note that if **watchdog** option is also specified vpmu will be turned off.
 
 *Warning:*
-As the BTS virtualisation is not 100% safe and because of the nehalem quirk
-don't use the vpmu flag on production systems with Intel cpus!
+As the virtualisation is not 100% safe, don't use the vpmu flag on
+production systems (see Xen Security Advisory 163,
+http://xenbits.xen.org/xsa/advisory-163.html)!
 
 ### watchdog
 > `= force | <boolean>`