xenapi: simplify PV vs HVM selection logic

The logic to inspect a disk to determine if an image is PV or HVM has
been removed. This change follows on from that simplification.

As before, if I user wants to use PV mode, or HVM mode, then can set
vm_mode=xen or vm_mode=hvm, respectively. This will override any other
way of selecting the mode.

For consistency, we now check the os_type to be either windows or linux,
which, without vm_mode, chooses HVM or XEN mode respectively.

For backwards compatibility, all VHD and DISK type images default to
vm_mode=xen.

In all other cases, we default to HVM, as this is the safest option
that works with most images.

Fixes bug 1206828
DocImpact
Change-Id: Iab2ffdc7ad396b853c7aed20d46bf12320b76c21
Merge "xenapi: simplify PV vs HVM selection logic"