pcap: Replace pcapm() with pcap_multiple()

pcapm() captures multiple frames from a msghdr, however the only thing it
cares about in the msghdr is the list of buffers, where it assumes there is
one frame to capture per buffer.  That's what we want for its single caller
but it's not the only obvious choice here (one frame per msghdr would
arguably make more sense in isolation).  In addition pcapm() has logic
that only makes sense in the context of the passt specific path its called
from: it skips the first 4 bytes of each buffer, because those have the
qemu vnet_len rather than the frame proper.

Make this clearer by replacing pcapm() with pcap_multiple() which more
explicitly takes one struct iovec per frame, and parameterizes how much of
each buffer to skip (i.e. the offset of the frame within the buffer).

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

tcp.c

@@ -1472,7 +1472,8 @@ static void tcp_l2_buf_flush(struct ctx *c, struct msghdr *mh,
 		}
 	}
 	*buf_used = *buf_bytes = 0;
-	pcapm(mh);
+
+	pcap_multiple(mh->msg_iov, mh->msg_iovlen, sizeof(uint32_t));
 }
 
 /**