xenapi: fix support for iso boot

Fixes bug 1154731 that stated the iso support was broken.

There were several issues around a vdis list without a 'root' disk.

Because the ISO boot was HVM, only 0-3 devices will work, so, the
devices used have been re-ordered to ensure ISOs can be read
by a HVM guest without PV tools. Once PV tools are present, a
HVM guest will be able to access an ephemeral or swap disk.

Change-Id: I4b57f70301a4256fa975323d7964fde19e8d4508
Merge "xenapi: fix support for iso boot"