Fix parsing of features in detect_vm_xen_dom0 (#7890)

Use sscanf instead of the built-in safe_atolu because the scanned string
lacks the leading "0x", it is generated with snprintf(b, "%08x", val).
As a result strtoull handles it as octal, and parsing fails.

The initial submission already used sscanf, then parsing was replaced by
safe_atolu without retesting the updated PR.

Fixes 575e6588 ("virt: use XENFEAT_dom0 to detect the hardware domain
(#6442, #6662) (#7581)")