util/qemu-sockets.c: Support specifying IPv4 or IPv6 in socket_dgram()

Currently you can specify whether you want a UDP chardev backend
to be IPv4 or IPv6 using the ipv4 or ipv6 options if you use the
QemuOpts parsing code in inet_dgram_opts(). However the QMP struct
parsing code in socket_dgram() doesn't provide this flexibility
(which in turn prevents us from converting the UDP backend handling
to the new style QAPI framework).

Use the existing inet_addr_to_opts() function to convert the
remote->inet address to option strings; this handles ipv4 and
ipv6 flags as well as host and port. (It will also convert any
'to' specification, which is harmless as it is ignored in this
context.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1409653457-27863-3-git-send-email-peter.maydell@linaro.org
