bug fix: Properly generate guest disks on VMTemplate

Commits cdd146a9 and 749abbf8 changed the way VMTemplate generates the
guest disks XML but, due a merge issue, they did not update the
to_vm_xml() function that was pointing to non-existing functions:
_get_iscsi_disks_xml() and _get_scsi_disks_xml()

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>