Add note about config drive, backing storage

From Grizzly, config drive is now supported well in Xen.

However, when using it, it is important to adjust the
 xenapi_disable_agent option, which this patch adds a note
about.

patchset2 adds a new config example snippet for backing storage

patchset3 addresses Diane's comments

patchset4 fixes syntax

fixes bug 1095506
fixes bug 1085404

Cherry-picked from https://review.openstack.org/35735

Change-Id: Ica515d3d2613d6b850bf7693715cc59dff73c5b4