Interpret BDM None size field as 0 on compute side

This patch makes sure that even if we receive an ephemeral or swap BDM
without size information - it is defaulted to 0 by the compute service
prior to booting the VM, as the size is expected to be present and an
integer number by the driver code that uses it.

Closes-bug: #1218861

Change-Id: Ic6fc15d1cf876aa4fd99e17c582c1ab55e3a18c1
Merge "Interpret BDM None size field as 0 on compute side"