iscsi: Adjust error message for findStorageSources backend

iscsi: Adjust error message for findStorageSources backend

The virStorageBackendISCSIFindPoolSources API only needs the 'host' name
in order to discover iSCSI pools, it returns the various device paths.
On input, it's also possible to further restrict a search by providing the
port attribute for the host element and the (undocumented) initiator element.

For example:

$  virsh find-storage-pool-sources-as iscsi
error: Failed to find any iscsi pool sources
error: invalid argument: hostname and device path must be specified for iscsi sources

$ virsh find-storage-pool-sources-as iscsi 192.168.122.1
<sources>
  <source>
    <host name='192.168.122.1' port='3260'/>
    <device path='iqn.2013-12.com.example:iscsi-chap-lclpool'/>
  </source>
</sources>