pcap: Don't reinitialise packet capture if we already have one

If the guest disconnects, and a given name (without timestamp) for
the pcap file is passed, we would otherwise lose the packets
captured until that point.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pcap.c

@@ -162,7 +162,7 @@ void pcap_init(struct ctx *c, int index)
 	struct tm *tm;
 
 	if (pcap_fd != -1)
-		close(pcap_fd);
+		return;
 
 	if (!*c->pcap)
 		return;