Make hw/vfio configurable and add new CONFIG_VFIO_* to the
default-configs/s390x*-softmmu.mak.  This allow a finer-grain
selection of the various VFIO backends.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190202072456.6468-28-yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>