qemuBuildHostdevSCSIAttachPrepare: Propagate 'readonly' flag also for iSCSI

qemuBuildHostdevSCSIAttachPrepare: Propagate 'readonly' flag also for iSCSI

The 'readonly' hostdev property is stored separately from the
virStorageSource as some hostdevs are not described by a virStorage
source. We need to propagate the flag to the virStorage source also for
iSCSI backends as it's used to generate the backend properties.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1868856

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>