qemu: Split out initialization of secrets for 'iscsi' hostdevs

qemu: Split out initialization of secrets for 'iscsi' hostdevs

Currently we don't have infrastructure to remember the secret aliases
for hostdevs. Since an upcoming patch is going to change aliases for
the disks, initialize the iscsi hostdevs separately so that we can keep
the alias. At the same time let's use qemuAliasForSecret instead of
qemuDomainGetSecretAESAlias when unplugging the iscsi hostdev.

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