resume guests on libvirt host reboot fails for instances created from multi-part image

[Alex Ermolov]

This is backport for upstream fix for: https://bugs.launchpad.net/nova/+bug/1195884

Original description:
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If an instance is created from a multi-part image (i.e. separate kernel and ramdisk files) , and resume_guests_state_on_host_boot=True the resulting _hard_reboot will fail and put the instance into an error state.

This is because the code in _hard_reboot that checks the image files, _create_images_and_backing() always tries to reload these from Glance.

In a normal reboot this results in an unnecessary image download from Glance.

In the case of reboot following a host boot the download fails because the context at this point does not have credentials to be able to access Glance.

Possible fix is to:
- Check files exist rather than always trying to download them
- Provide in conf credentials of a user that has read access to all images in Glance than can be used in this context