XenAPINFS: fix capacity reporting

Fixes bug 1129056

XenAPINFS did not implement the get_volume_stats method, thus
CapacityFilter fitered out the host, thus no volume was scheduled. This
fix is working around this problem quickly, by reporting an 'unknown'
capacity.

Change-Id: I1dc5327e420a3316201318d52f602ced071d459b