VMware: prevent exception with migrate_disk_and_power_off

Commit 8e4a9156f4dccf003970848c28b8a9d15c55212d ensured that
all instance operations that do not require a cluster or
volume will make use of the base _vmops class.

The method migrate_disk_and_power_off was missed.

Change-Id: Ibfdd9407905acc5e9a3e3a97a76fb2e35a68e817
Merge "VMware: prevent exception with migrate_disk_and_power_off"