tap: Extend tap_send_frames() to allow multi-buffer frames

tap_send_frames() takes a vector of buffers and requires exactly one frame
per buffer.  We have future plans where we want to have multiple buffers
per frame in some circumstances, so extend tap_send_frames() to take the
number of buffers per frame as a parameter.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Improve comment to rembufs calculation]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tap.h

@@ -73,7 +73,8 @@ void tap_icmp6_send(const struct ctx *c,
 		    const struct in6_addr *src, const struct in6_addr *dst,
 		    const void *in, size_t len);
 int tap_send(const struct ctx *c, const void *data, size_t len);
-size_t tap_send_frames(const struct ctx *c, const struct iovec *iov, size_t n);
+size_t tap_send_frames(const struct ctx *c, const struct iovec *iov,
+		       size_t bufs_per_frame, size_t nframes);
 void eth_update_mac(struct ethhdr *eh,
 		    const unsigned char *eth_d, const unsigned char *eth_s);
 void tap_listen_handler(struct ctx *c, uint32_t events);