target-i386: Set level=4 on Conroe/Penryn/Nehalem

target-i386: Set level=4 on Conroe/Penryn/Nehalem

The CPUID level value on Conroe, Penryn, and Nehalem are too low. This
causes at least one known problem: the -smp "threads" option doesn't
work as expect if level is < 4, because thread count information is
provided to the guest on CPUID[EAX=4,ECX=2].EAX

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