target-i386: disable pv eoi to fix migration across QEMU versions

We have a problem with how we handle migration with KVM paravirt features.
We unconditionally enable paravirt features regardless of whether we know how
to migrate them.

We also don't tie paravirt features to specific machine types so an old QEMU on
a new kernel would expose features that never existed.

The 1.2 cycle is over and as things stand, migration is broken.  Michael has
another series that adds support for migrating PV EOI and attempts to make it
work correctly for different machine types.

After speaking with Michael on IRC, we agreed to take this patch plus 1 & 4
from his series.  This makes sure QEMU can migrate PV EOI if it's enabled, but
does not enable it by default.

This also means that we won't unconditionally enable new features for guests
future proofing us from this happening again in the future.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
