virt-driver-qemu.m4: Make qemu driver optional

The basic idea of our configure script is to probe for things
rather than have them enabled by default. This is even more
visible in the next commit where configure fails if qemu driver
is enabled but no yajl is found.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJiri Denemark <jdenemar@redhat.com>

m4/virt-driver-qemu.m4

@@ -18,7 +18,7 @@ dnl <http://www.gnu.org/licenses/>.
 dnl
 
 AC_DEFUN([LIBVIRT_DRIVER_ARG_QEMU], [
-  LIBVIRT_ARG_WITH_FEATURE([QEMU], [QEMU/KVM], [yes])
+  LIBVIRT_ARG_WITH_FEATURE([QEMU], [QEMU/KVM], [check])
   LIBVIRT_ARG_WITH([QEMU_USER], [username to run QEMU system instance as],
                    ['platform dependent'])
   LIBVIRT_ARG_WITH([QEMU_GROUP], [groupname to run QEMU system instance as],