core: Add validation for DC's compatability.

Move storage pool validation for DC before fetching the disks when
creating a snapshot.
Since VM without snapshots will not get to this validation.

Change-Id: I77e301643b3ac9789cc5eb7527b6a5a4a0aac6be
Related-to: https://bugzilla.redhat.com/888862
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>