Comment 7 for bug 1713857

Reviewed: https://review.openstack.org/523911
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=aa0e4ebd2c02c042bbe2c0d58e690e3e43b04a17
Submitter: Zuul
Branch: stable/ocata

commit aa0e4ebd2c02c042bbe2c0d58e690e3e43b04a17
Author: melanie witt <email address hidden>
Date: Tue Aug 29 22:53:44 2017 +0000

    Save updated libvirt domain XML after swapping volume

    When a user calls the volume-update API, we swap_volume in the libvirt
    driver from the old volume attachment to the new volume attachment.
    Currently, we're saving the domain XML with the old configuration prior
    to updating the volume and upon a soft-reboot request, it results in an
    error:

      Instance soft reboot failed: Cannot access storage file <old path>

    and falls back to a hard reboot, which is like pulling the power cord,
    possibly resulting in file system inconsistencies.

    This changes to saving the new, updated domain XML after the volume
    swap.

    Closes-Bug: #1713857

    NOTE(melwitt): The assert in test_swap_volume_block_old_libvirt had to
    be modified to account for the additional XMLDesc call from the bug
    fix. The test_swap_volume_block_old_libvirt doesn't exist in Pike, so
    that's why we didn't hit this in the master patch.

    Change-Id: I166cde5ad8b00699e4ec02609f0d7b69236d855d
    (cherry picked from commit 5b008c6540f948d28dc50a0ef84095ebd96d198d)