Updated can_set_mount_point default in docs

The can_set_mount_point default has been changed from True to False,
reflecting that only the Xen hypervisor currently supports the use of
this option. This patch updates the documentation to reflect the
change.

The default changed in commit: d34d7904eed431937b3d75c11da7743767ce9551

Change-Id: I4522718cb7335a1f13557acc0bad1e8f407571ad
Closes-Bug: #1255136
Merge "Updated can_set_mount_point default in docs"