x86/svm: Intercept Bus Locks for HVM guests

Configure the Bus Lock intercept when supported by hardware (Zen5 and
later). The VMCB counter is initialised to zero so it fires upon the first
instruction that locks the bus. On the #VMEXIT handler that counter is set to
1 because it has fault behaviour and the offending instruction needs to
re-execute.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

CHANGELOG.md

@@ -9,6 +9,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Changed
 
 ### Added
+ - On x86:
+   - Support for Bus Lock Threshold on AMD Zen5 and later CPUs, used by Xen to
+     mitigate (by rate-limiting) the system wide impact of an HVM guest
+     misusing atomic instructions.
 
 ### Removed
  - On x86: