Storage volume upload: Dispatch volume create to right handler

As we are starting to support upload and download to create volume,
they need to be distinguished from previous creating through libvirt api.
Adding a dispatcher to support this.

Also update the jsonschema to add capacity parameter.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Storage volume upload: Change storagevolumes to AsyncCollection

Because storage volume create will include upload and download,
storagevolumes will not present before async task's finished.
so change storage volumes to async collection.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>