core: Standardize StoragePoolValidator

Standardized StoragePoolValidator to look like all the other validators.

The constructor now accepts only a storage_pool object and all the
validation methods return ValidationResult objects with the
VdcBllMessage instead of returning a boolean and adding the
VdcBllMessage to a list.

Change-Id: I79ede9de954b209c9c27451e430e9d6219cded0f
Signed-off-by: Allon Mureinik <amureini@redhat.com>