tap: Implement tap_send() "slow path" in terms of fast path

Most times we send frames to the guest it goes via tap_send_frames().
However "slow path" protocols - ARP, ICMP, ICMPv6, DHCP and DHCPv6 - go
via tap_send().

As well as being a semantic duplication, tap_send() contains at least one
serious problem: it doesn't properly handle short sends, which can be fatal
on the qemu socket connection, since frame boundaries will get out of sync.

Rewrite tap_send() to call tap_send_frames().  While we're there, rename it
tap_send_single() for clarity.

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

tap.h

@@ -72,7 +72,7 @@ void tap_udp6_send(const struct ctx *c,
 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);
+void tap_send_single(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 bufs_per_frame, size_t nframes);
 void eth_update_mac(struct ethhdr *eh,