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>

pcap.h

@@ -7,7 +7,8 @@
 #define PCAP_H
 
 void pcap(const char *pkt, size_t len);
-void pcap_multiple(const struct iovec *iov, unsigned int n, size_t offset);
+void pcap_multiple(const struct iovec *iov, size_t frame_parts, unsigned int n,
+		   size_t offset);
 void pcap_init(struct ctx *c);
 
 #endif /* PCAP_H */