x86/cpuid: Move all xstate leaf handling into guest_cpuid()

x86/cpuid: Move all xstate leaf handling into guest_cpuid()

The xstate union now contains sanitised values, so it can be handled fully in
the non-legacy path.

c/s 1c0bc709d "x86/cpuid: Perform max_leaf calculations in guest_cpuid()"
accidentally introduced a boundary error for the subleaf check, although it
was masked by the correct logic in the legacy path.

Two dynamic adjustments need making, but a TODO and BUILD_BUG_ON() are left to
cover a latent bug which will present itself when Xen starts supporting XSS
states for guests.

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