While Xen only has a single paravirt console, UML, and
QEMU both support multiple paravirt consoles. The LXC
driver can also be trivially made to support multiple
consoles. This patch extends the XML to allow multiple
<console> elements in the XML. It also makes the UML
and QEMU drivers support this config.

* src/conf/domain_conf.c, src/conf/domain_conf.h: Allow
  multiple <console> devices
* src/lxc/lxc_driver.c, src/xen/xen_driver.c,
  src/xenxs/xen_sxpr.c, src/xenxs/xen_xm.c: Update for
  internal API changes
* src/security/security_selinux.c, src/security/virt-aa-helper.c:
  Only label consoles that aren't a copy of the serial device
* src/qemu/qemu_command.c, src/qemu/qemu_driver.c,
  src/qemu/qemu_process.c, src/uml/uml_conf.c,
  src/uml/uml_driver.c: Support multiple console devices
* tests/qemuxml2xmltest.c, tests/qemuxml2argvtest.c: Extra
  tests for multiple virtio consoles. Set QEMU_CAPS_CHARDEV
  for all console /channel tests
* tests/qemuxml2argvdata/qemuxml2argv-channel-virtio-auto.args,
  tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.args
  tests/qemuxml2argvdata/qemuxml2argv-console-virtio.args: Update
  for correct chardev syntax
* tests/qemuxml2argvdata/qemuxml2argv-console-virtio-many.args,
  tests/qemuxml2argvdata/qemuxml2argv-console-virtio-many.xml: New
  test file