core: Calculate storage format when adding an SD

Removed the default V1 format from StorageDomainStatic's constructor so
it can be calculated when adding a domain without specifying the format
via REST.

The logic goes as follows:
- If the format is specified (as is the case in all the edit scenarios
  as well as when adding a new domain via the GUI), leave it alone
- If it's a data domain, set the format to the format required by the
  pool's version.
  Note: Creating a domain by a host not attached to a pool is currently
  NOT supported. This patch does not change that behavior, nor does it
  handle this situation.
- If it's not a data domain, use V1 (i.e., preserve the old behavior).

Change-Id: If469bb655e9a65e2d0afcee164655fa0bdfa5d99
Bug-Url: https://bugzilla.redhat.com/1120712
Signed-off-by: Allon Mureinik <amureini@redhat.com>