As the description of removing CDROM media from
  http://wiki.libvirt.org/page/QEMUSwitchToLibvirt#eject_DEV

Add flag 'VSH_OFLAG_EMPTY_OK' to the option 'source' of attach-disk

Then avoid outputting <source> in the XML if 'source' was empty,
rather than trusting libvirt domain_conf.c to understand an empty
string.
Signed-off-by: NXu He Jie <xuhj@linux.vnet.ibm.com>
Signed-off-by: NEric Blake <eblake@redhat.com>