target-i386: Enable "check" mode by default

Current default behavior of QEMU is to silently disable features that
are not supported by the host when a CPU model is requested in the
command-line. This means that in addition to risking breaking guest ABI
by default, we are silent about it.

I would like to enable "enforce" by default, but this can easily break
existing production systems because of the way libvirt makes assumptions
about CPU models today (this will change in the future, once QEMU
provide a proper interface for checking if a CPU model is runnable).

But there's no reason we should be silent about it. So, change
target-i386 to enable "check" mode by default so at least we have some
warning printed to stderr (and hopefully logged somewhere) when QEMU
disables a feature that is not supported by the host system.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>