pcap: Handle short writes in pcap_frame()

Currently pcap_frame() assumes that if write() doesn't return an error, it
has written everything we want.  That's not necessarily true, because it
could return a short write.  That's not likely to happen on a regular file,
but there's not a lot of reason not to be robust here; it's conceivable we
might want to direct the pcap fd at a named pipe or similar.

So, make pcap_frame() handle short frames by using the write_remainder()
helper.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Formatting fix, and avoid gcc warning in pcap_frame()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tap.c

@@ -433,7 +433,7 @@ size_t tap_send_frames(const struct ctx *c, const struct iovec *iov, size_t n)
 	if (m < n)
 		debug("tap: failed to send %zu frames of %zu", n - m, n);
 
-	pcap_multiple(iov, m, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
+	pcap_multiple(iov, 1, n, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
 
 	return m;
 }