This change adds the ability to extend 'in-use' volume.
Once the volume size is extended, Nova is informed of the size change
through the external-event extension so the virt driver can perform
the appropriate actions for the host and guest to detect the new volume size.
Tempest related patches:
1. https://review.openstack.org/#/c/480746/
2. https://review.openstack.org/#/c/480778/
Depends-On: If10cffd0dc4c9879f6754ce39bee5fae1d04f474
Blueprint: extend-attached-volume
Co-Authored-By: TommyLike <tommylikehu@gmail.com>
APIImpact
Change-Id: I60c8ea9eb0bbcfe41f5f0a30ed8dc67bdcab3ebc