https://bugzilla.redhat.com/show_bug.cgi?id=1200206

Commit id '1b4eaa61' added the ability to have a mode='direct' for
an iscsi disk volume.  It relied on virStorageTranslateDiskSourcePool
in order to copy any disk source pool authentication information to
the direct disk volume, but it neglected to also copy the 'secrettype'
field which ends up being used in the domain volume formatting code.
Adding a secrettype for this case will allow for proper formatting later
and allow disk snapshotting to work properly

Additionally libvirtd restart processing would fail to find the domain
since the translation processing code is run after domain xml processing,
so handle the the case where the authdef could have an empty secrettype
field when processing the auth and additionally ignore performing the
actual and expected auth secret type checks for a DISK_VOLUME since that
data will be reassembled later during translation processing of the
running domain.