udp: Use tap_send_frames()

To send frames on the tap interface, the UDP uses a fairly complicated two
level batching.  First multiple frames are gathered into a single "message"
for the qemu stream socket, then multiple messages are send with
sendmmsg().  We now have tap_send_frames() which already deals with sending
a number of frames, including batching and handling partial sends.  Use
that to considerably simplify things.

This does make a couple of behavioural changes:
  * We used to split messages to keep them under 32kiB (except when a
    single frame was longer than that).  The comments claim this is
    needed to stop qemu from closing the connection, but we don't have any
    equivalent logic for TCP.  I wasn't able to reproduce the problem with
    this series, although it was apparently easy to reproduce earlier.

    My suspicion is that there was never an inherent need to keep messages
    small, however with larger messages (and default kernel buffer sizes)
    the chances of needing more than one resend for partial send()s is
    greatly increased.  We used not to correctly handle that case of
    multiple resends, but now we do.

  * Previously when we got a partial send on UDP, we would resend the
    remainder of the entire "message", including multiple frames.  The
    common code now only resends the remainder of a single frame, simply
    dropping any frames which weren't even partially sent.  This is what
    TCP always did and is probably a better idea for UDP too.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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