tools/libx[cl]: Plumb bool restore down into xc_cpuid_apply_policy()

tools/libx[cl]: Plumb bool restore down into xc_cpuid_apply_policy()

In order to safely disable some features by default, without breaking
migration from 4.13 or older, the CPUID logic needs to distinguish the two
cases.

Plumb a restore boolean down from the two callers of libxl__cpuid_legacy() all
the way down into xc_cpuid_apply_policy().

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>