VMware: Implement backup/restore for VMDK driver

VMware: Implement backup/restore for VMDK driver

Currently, backup-create and backup-restore operations are not
supported for volumes created by the VMDK driver. This change
implements backup_volume and restore_backup APIs for VMDK driver.

DocImpact
    Added a new config option 'vmware_tmp_dir' which specifies
    the directory in Cinder host to use for storing temporary
    virtual disk files during backup/restore.

Implements: blueprint vmdk-backup
Change-Id: Ib19a699f71701c2034e77b8e6b50a8a3295a542b