x86/emul: Don't use the ->cpuid() hook for feature checks

x86/emul: Don't use the ->cpuid() hook for feature checks

For a release build of xen, this removes nearly 5k of code volume, and removes
a function pointer call from every instantiation.

  add/remove: 0/1 grow/shrink: 0/3 up/down: 0/-4822 (-4822)
  Function                                     old     new   delta
  adjust_bnd                                   260     244     -16
  x86_decode                                  8915    8890     -25
  vcpu_has.isra                                129       -    -129
  x86_emulate                               130040  125388   -4652
  Total: Before=3326565, After=3321743, chg -0.14%

Note that one corner case changes.  At the moment, it is possible for an
entity making direct DOMCTL_set_cpuid hypercalls to construct a policy with
max_leaf < 7, but feature bits set in leaf 7.  By default, libxc and libxl
don't do this, and the result is properly bounded by what the hardware is
capable of (so we won't start trying to use instructions which don't exist in
the CPU).

Previously, the cpuid() hook would end up hiding these features, but they may
still be set cpuid_policy, and therefore might start being accepted by
x86_emulate().

This corner case will be fixed by the in-progress DOMCTL_set_cpu_policy work,
and a guest would only encounter the corner case if it was constructed in a
non-standard manner, and if tried using instruction which it couldn't see
CPUID feature bits for.  As such, it isn't a corner case which we need to
worry about.

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