target-i386: Set model=6 on qemu64 & qemu32 CPU models

There's no Intel CPU with family=6,model=2, and Linux and Windows guests
disable SEP when seeing that combination due to Pentium Pro erratum #82.

In addition to just having SEP ignored by guests, Skype (and maybe other
applications) runs sysenter directly without passing through ntdll on
Windows, and crashes because Windows ignored the SEP CPUID bit.

So, having model > 2 is a better default on qemu64 and qemu32 for two
reasons: making SEP really available for guests, and avoiding crashing
applications that work on bare metal.

model=3 would fix the problem, but it causes CPU enumeration problems
for Windows guests[1]. So let's set model=6, that matches "Athlon
(PM core)" on AMD and "P2 with on-die L2 cache" on Intel and it allows
Windows to use all CPUs as well as fixing sysenter.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=508623

Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>