Fix booting a raw image on XenServer
Fix booting a raw image on XenServer

Fixes bug 1055413.

ImageType.from_string removed, as it was only used by its test.
Introduced ImageType.get_role, that returns the image's role within the
VM. In case of DISK_* image types, the role is always 'root'. Role
should be the key when accessing the vdis dictionary.

Change-Id: Ieb952a9246d06a8280510113a98b888237abcc65

n0ano/gantt,n0ano/gantt