qemu_vm: Fix enable/disable_kvm option

When we set disable_kvm to 'yes', we obviously don't want
-enable-kvm set regardless of the value of enable_kvm.

So let's fix that. Now you can just set

disable_kvm = yes

And everything else will be taken care of.

CC: Eduardo Habkost <ehabkost@redhat.com>
CC: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>