Migrate-volume-block-migration

Add Support for Volume migration during Live (block) migration

External Specification:

Add --block_device_mapping parameter to "nova live-migration --block_migrate" command.

The entire format of the command will be like this

`nova live-migration --block-migrate --block-device-mapping vda=1:::0, --block-device-mapping vdb=...(all attached EBS volumes) <instance_name> <name_of_destination_compute_node>`

Administrator has to specify --block_device_mapping parameters for all volumes attached to the instance.

Follow the existing format to specify block device mapping

`--block-device-mapping <device_name>=<a>::<c>:<d>`

   <device_name>: device name referred from the instance

   <a>: destination volume uuid (Note: MUST specify an existing volume, must be same size to the source volume)

   <b>: N/A

   <c>: N/A or size of the volume

   <d>: 0:Do not delete the source volume, 1:delete the source volume

Note: Volumes on the destination volume node need to be created before the migration takes place. The migration could ensure copying the data onto the destination volumes.