xenapi: determine_is_pv should be fixed to have a safer default of HVM

[John Garbutt]

The auto PV detection was removed as part of this bug:
https://code.launchpad.net/bugs/1196570

We should consider removing the check for the disk type and simply have a check like:

If os_type = linux then
  PV
else
  HVM

Defaulting to PV is a bit unsafe, as HVM is more likely to work for things that have no vm_mode (already checked before this code, should probably try and pull that code into the above method) and no os_type flags.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/46498

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/46498
Committed: http://github.com/openstack/nova/commit/2cc2c786f4525acc73d1e61f4706ed0aaa453c25
Submitter: Jenkins
Branch: master

commit 2cc2c786f4525acc73d1e61f4706ed0aaa453c25
Author: John Garbutt <email address hidden>
Date: Fri Sep 13 14:30:37 2013 +0100

    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