tevent: traverse the ev->fd_event list instead of the poll_ev->fds array

tevent: traverse the ev->fd_event list instead of the poll_ev->fds array

That has the same complexity but simplifies the code.

It also seems to fix a real problem, which was found
by the following test failure in Samba's autobuild,
while removing the epoll support from the "standard" backend.

    [402/1555 in 19m8s] samba3.raw.composite(plugin_s4_dc)
    smbtorture 4.1.0pre1-DEVELOPERBUILD
    Using seed 1361531675
    Testing savefile
    Testing parallel fetchfile with 10 ops
    waiting for completion
    maximum runtime exceeded for smbtorture - terminating
    UNEXPECTED(error): samba3.raw.composite.fetchfile (subunit.RemotedTestCase)(plugin_s4_dc)
    REASON: _StringException: _StringException: was started but never finished!
    UNEXPECTED(error): samba3.raw.composite.fetchfile(plugin_s4_dc) (subunit.RemotedTestCase)
    REASON: was started but never finished!

Signed-off-by: Stefan Metzmacher <metze@samba.org>