libvirt: make live_migration_uri flag dependent on virt_type

[OpenStack Infra]

https://review.openstack.org/175780
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commit 3159c8fd5bea80c820e58bd38d96f5f8fe8f4503
Author: Alvaro Lopez Garcia <email address hidden>
Date: Tue Apr 21 10:39:30 2015 +0200

    libvirt: make live_migration_uri flag dependent on virt_type

    The default value for the "live_migration_uri" flag should be
    dependent on the "virt_type" flag, as the "connection_uri" flag is. This
    way, an operator can set the "virt_type" flag without the need to check
    for each individual uri.

    DocImpact: Changed the default value of the "live_migration_uri" flag,
    that now is dependent on the "virt_type".
    Closes-Bug: #1298242
    Change-Id: Id54f7bdfa14a19da41da554b13ba9496ee525c71

[Sean Dague (sdague)]

The manual update required here should be that the live_migration_uri config option has defaults that are automatically selected based on libvirt driver backend (instead of one that only works for qemu/kvm). Less people should need to override this value now.

[Anne Gentle (annegentle)]

Because this is a configuration option, I hope that the automation for configuration options docs would simply pick it up. That said, do we need to write something in addition to what will be scraped?

[Atsushi SAKAI (sakaia)]

@Anne
I think update the help message as follows for Config reference.
If other discription of live_migration_url exists, it should be removed since a few people should use this parameter.
https://github.com/openstack/nova/blob/5ae39f70f5b202bffd3165ecb9082d4a4f25090b/nova/virt/libvirt/driver.py#L165

https://github.com/openstack/nova/blob/5ae39f70f5b202bffd3165ecb9082d4a4f25090b/nova/virt/libvirt/driver.py#L849