Ensure we always read a full buffer in tunnelled migration

The 'nbytes' variable was not re-initialized to the
buffer size on each iteration of the tunnelled migration
loop. While saferead() will ensure a full read, except
on EOF, it is clearer to use the real buffer size

* src/qemu/qemu_migration.c: Always read full buffer of data

src/qemu/qemu_migration.c

@@ -1300,7 +1300,7 @@ static int doTunnelSendAll(virStreamPtr st,
     }
 
     for (;;) {
-        nbytes = saferead(sock, buffer, nbytes);
+        nbytes = saferead(sock, buffer, TUNNEL_SEND_BUF_SIZE);
         if (nbytes < 0) {
             virReportSystemError(errno, "%s",
                                  _("tunnelled migration failed to read from qemu"));