cluster/afr: get virtual-xattrs only on valid xdata.

Change-Id: I61fd891435faced25b2bdf617ec07a8af8ef057d
BUG: 1046853
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/6605
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>