target-i386: CPUID: return highest basic leaf if eax > cpuid_xlevel

This fixes a subtle bug. A bug that probably won't cause trouble for any
existing OS, but a bug anyway:

Intel SDM Volume 2, CPUID Instruction states:

> Two types of information are returned: basic and extended function
> information. If a value entered for CPUID.EAX is higher than the maximum
> input value for basic or extended function for that processor then the
> data for the highest basic information leaf is returned. For example,
> using the Intel Core i7 processor, the following is true:
>
>   CPUID.EAX = 05H (* Returns MONITOR/MWAIT leaf. *)
>   CPUID.EAX = 0AH (* Returns Architectural Performance Monitoring leaf. *)
>   CPUID.EAX = 0BH (* Returns Extended Topology Enumeration leaf. *)
>   CPUID.EAX = 0CH (* INVALID: Returns the same information as CPUID.EAX = 0BH. *)
>   CPUID.EAX = 80000008H (* Returns linear/physical address size data. *)
>   CPUID.EAX = 8000000AH (* INVALID: Returns same information as CPUID.EAX = 0BH. *)

AMD's CPUID Specification, on the other hand, is less specific:

> The CPUID instruction supports two sets or ranges of functions,
> standard and extended.
>
> • The smallest function number of the standard function range is
>   Fn0000_0000. The largest function num- ber of the standard function
>   range, for a particular implementation, is returned in CPUID
>   Fn0000_0000_EAX.
>
> • The smallest function number of the extended function range is
>   Fn8000_0000. The largest function num- ber of the extended function
>   range, for a particular implementation, is returned in CPUID
>   Fn8000_0000_EAX.
>
> Functions that are neither standard nor extended are undefined and
> should not be relied upon.

QEMU's behavior matched Intel's specification before, but this was
changed by commit b3baa152aaef1905876670590275c2dd0bbb088c. This patch
restores the behavior documented by Intel when cpuid_xlevel2 is 0.

The existing behavior when cpuid_xlevel2 is set (falling back to
level=cpuid_xlevel) is being kept, as I couldn't find any public
documentation on the CPUID 0xC0000000 function range on Centaur CPUs.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
