Fix QEMU -vnc arg generation with raw IPv6 addresses

Fix QEMU -vnc arg generation with raw IPv6 addresses

Since -vnc uses ':' to separate the address from the port, raw
IPv6 addresses need to be escaped like [addr]:port

* src/qemu/qemu_command.c: Escape raw IPv6 addresses with []
* tests/qemuxml2argvdata/qemuxml2argv-graphics-vnc.args,
  tests/qemuxml2argvdata/qemuxml2argv-graphics-vnc.xml: Tweak
  to test Ipv6 escaping
* docs/schemas/domain.rng: Allow Ipv6 addresses, or hostnames
  in <graphics> listen attributes