virt-yajl: Fix detection of yajl requirements

Running the output of qemu -help doesn't make any sense.  We should be
looking for libvirt being mentioned in the output.  This worked by
accident, let's make it work as expected it to.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

m4/virt-yajl.m4

@@ -26,7 +26,7 @@ AC_DEFUN([LIBVIRT_CHECK_YAJL],[
     AC_PATH_PROGS([QEMU], [qemu-kvm qemu kvm qemu-system-x86_64],
                   [], [$PATH:/usr/bin:/usr/libexec])
     if test -x "$QEMU"; then
-      if `$QEMU -help | grep libvirt` >/dev/null; then
+      if $QEMU -help 2>/dev/null | grep -q libvirt; then
         with_yajl=yes
       else
         [qemu_version_sed='s/.*ersion \([0-9.,]*\).*/\1/']