core: Fix CDA for AddVmCommand

The command allowed a user to add a VM using REST before the DC was initialized,
this VM caused problems in some flows. Added a check for the storage pool status
that will cause the command to fail when the DC is down.

Change-Id: I2586b0026c85f053b6cd0aebeb555d760afc0937
Bug-Url: https://bugzilla.redhat.com/1157229
Signed-off-by: Eldan Shachar <eshachar@redhat.com>