configure: report missing libraries for virtfs

Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -2915,7 +2915,8 @@ if test "$softmmu" = yes ; then
       tools="$tools fsdev/virtfs-proxy-helper\$(EXESUF)"
     else
       if test "$virtfs" = yes; then
-        feature_not_found "virtfs"
+        echo "VirtFS is supported only on Linux and requires libcap-devel and libattr-devel"
+        exit 1
       fi
       virtfs=no
     fi