tools/libxc: Fix the reported max_leaf values for PV guests

tools/libxc: Fix the reported max_leaf values for PV guests

When iterating through CPUID leaves to generating a policy, libxc will clip
itself at the hardcoded maxima, meaning that no data outside of the hardcoded
maxima are provided to Xen (in turn, causing Xen to return zeros if these
leaves are requested.)

The HVM code also clips the max_leaf data reported to the guest, but the PV
side didn't.

This results in a PV guest using the emulated CPUID, or via Xen using CPUID
faulting, to observe a max_leaf higher than the toolstack wants, although with
zeros being returned in the intervening leaves.

Fix the PV side to behave like the HVM side, and clip the max_leaf values in
leaf 0 and 0x80000000.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>