xenapi: Document /boot/guest

Document the required Dom0 changes to support AMI type images.

Change-Id: Ic31ae7effae5e4d06580f159c664f21caea29078
Remove extraneous /admin/ webhelpDirname setting.

Change-Id: I578f1ff46d808584cfdaa2a67700f2b01cbc8366
Merge "xenapi: Document /boot/guest"