libxg: don't use max policy in xc_cpuid_xend_policy()

libxg: don't use max policy in xc_cpuid_xend_policy()

using max undermines the separation between default and max. For
example, turning off AVX512F on an MPX-capable system silently turns on
MPX, despite this not being part of the default policy anymore. Since
the information is used only for determining what to convert 'x' to (but
not to e.g. validate '1' settings), the effect of this change is
identical for guests with (suitable) "cpuid=" settings to that of the
changes separating default from max and then converting (e.g.) MPX from
being part of default to only being part of max for guests without
(affected) "cpuid=" settings.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>