conf: Use 0 instead of -1 as "unassigned" mtu value

On the command line -m 0 means "don't assign an MTU" (letting the guest use
its default.  However, internally we use (c->mtu == -1) to represent that
state.  We use (c->mtu == 0) to represent "the user didn't specify on the
command line, so use the default" - but this is only used during conf(),
never afterwards.

This is unnecessarily confusing.  We can instead just initialise c->mtu to
its default (65520) before parsing options and use 0 on both the command
line and internally to represent the "don't assign" special case.  This
ensures that c->mtu is always 0..65535, so we can store it in a uint16_t
which is more natural.

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

dhcp.c

@@ -417,7 +417,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
 		       &c->ip4.guest_gw, sizeof(c->ip4.guest_gw));
 	}
 
-	if (c->mtu != -1) {
+	if (c->mtu) {
 		opts[26].slen = 2;
 		opts[26].s[0] = c->mtu / 256;
 		opts[26].s[1] = c->mtu % 256;