This extends the SPICE XML to allow channel security options

    <graphics type='spice' port='-1' tlsPort='-1' autoport='yes'>
      <channel name='main' mode='secure'/>
      <channel name='record' mode='insecure'/>
    </graphics>

Any non-specified channel uses the default, which allows both
secure & insecure usage

* src/conf/domain_conf.c, src/conf/domain_conf.h,
  src/libvirt_private.syms: Add XML syntax for specifying per
  channel security options for spice.
* src/qemu/qemu_conf.c: Configure channel security with spice