target-i386: Fix including "host" in -cpu ? output

kvm_enabled() cannot be true at this point because accelerators are
initialized much later during init. Also, hiding this makes it very hard
to discover for users. Simply dump unconditionally if CONFIG_KVM is set.

Add explanation for "host" CPU type.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
