qemu: domain: Extract preparation of hostdev specific data to a separate function

qemu: domain: Extract preparation of hostdev specific data to a separate function

Historically we've prepared secrets for all objects in one place. This
doesn't make much sense and it's semantically more appealing to prepare
everything for a single device type in one place.

Move the setup of the (iSCSI|SCSI) hostdev secrets into a new function
which will be used to setup other things as well in the future.

This is a similar approach we do for disks.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>