Inspired by https://bugzilla.redhat.com/show_bug.cgi?id=615757

Add a new character device backend for virtio serial channels that
activates the QEMU spice agent on the main channel using the vdagent
spicevmc connection.  The <target> must be type='virtio', and supports
an optional name that specifies how the guest will see the channel
(for now, name must be com.redhat.spice.0).

<channel type='spicevmc'>
  <target type='virtio'/>
  <address type='virtio-serial' controller='1' bus='0' port='3'/>
</channel>

* docs/schemas/domain.rng: Support new XML.
* docs/formatdomain.html.in: Document it.
* src/conf/domain_conf.h (virDomainChrType): New enum value.
* src/conf/domain_conf.c (virDomainChr): Add spicevmc.
(virDomainChrDefParseXML, virDomainChrSourceDefParseXML)
(virDomainChrDefParseTargetXML): Parse and enforce proper use.
(virDomainChrSourceDefFormat, virDomainChrDefFormat): Format.
* src/qemu/qemu_command.c (qemuBuildChrChardevStr)
(qemuBuildCommandLine): Add qemu support.
* tests/qemuxml2argvtest.c (domain): New test.
* tests/qemuxml2argvdata/qemuxml2argv-channel-spicevmc.xml: New
file.
* tests/qemuxml2argvdata/qemuxml2argv-channel-spicevmc.args:
Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>