XenAPI: More operations with LVM-based SRs

Some operations require Files-based SRs (ext3/NFS) - such as resize
and VHD download.  However, if using 3-part images or RAW then most
operations can be successful with this simple fix.

Fixes bug 1162382

Change-Id: Ia3d1c64871d78ab1bf6e197edaeee170d2b5dea8
Merge "XenAPI: More operations with LVM-based SRs"