target-i386: Don't enable nested VMX by default

target-i386: Don't enable nested VMX by default

TCG doesn't support VMX, and nested VMX is not enabled by default in the
KVM kernel module.

So, there's no reason to have VMX enabled by default on the core2duo and
coreduo CPU models, today. Even the newer Intel CPU model definitions
don't have it enabled.

In this case, we need machine-type compat code, as people may be running
the older machine-types on hosts that had VMX nesting enabled.

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