[BLKTAP] Remove unnecessary timeout from tapdisk select() call.

[BLKTAP] Remove unnecessary timeout from tapdisk select() call.

tapdisk was using a 1msec select() timeout in its main event loop.  On
timeout, the loop does nothing except repeat, so the timeout achieves
nothing; worse, on a machine with HZ set to less than 1000, a msec
timeout returns immediately, sending tapdisk into a 100%-CPU-
utilisation loop.

Simply remove the timeout entirely; everything still works fine, and
much more efficiently when idle.

Signed-off-by: Stephen Tweedie <sct@redhat.com>