Fix detection of required RBD version

If building libvirt against Ubuntu precise, the librbd.h is lacking
many functions that libvirt expects. We have no version check, so
we were enabling RBD even though it cannot compile. This configure
check uses existance of 'rbd_get_features' as an identifier for the
min required version.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

m4/virt-storage-rbd.m4

@@ -28,12 +28,24 @@ AC_DEFUN([LIBVIRT_STORAGE_CHECK_RBD], [
     AC_CHECK_HEADER([rbd/librbd.h], [LIBRBD_FOUND=yes; break;])
 
     if test "$LIBRBD_FOUND" = "yes"; then
-      with_storage_rbd=yes
       LIBRBD_LIBS="-lrbd -lrados"
+
+      old_LIBS="$LIBS"
+      LIBS="$LIBS $LIBRBD_LIBS"
+      AC_CHECK_FUNCS([rbd_get_features],[],[LIBRBD_FOUND=no])
+      LIBS="$old_LIBS"
+    fi
+
+    if test "$LIBRDB_FOUND" = "yes"; then
+      with_storage_rbd=yes
       AC_DEFINE_UNQUOTED([WITH_STORAGE_RBD], [1],
                          [whether RBD backend for storage driver is enabled])
     else
-      with_storage_rbd=no
+      if test "$with_storage_rbd" = "yes"; then
+        AC_MSG_ERROR([You must install the librbd library & headers to compile libvirt])
+      else
+        with_storage_rbd=no
+      fi
     fi
   fi
   AM_CONDITIONAL([WITH_STORAGE_RBD], [test "$with_storage_rbd" = "yes"])