fdmon-poll: reset npfd when upgrading to fdmon-epoll

npfd keeps track of how many pollfds are currently being monitored. It
must be reset to 0 when fdmon_poll_wait() returns.

When npfd reaches a treshold we switch to fdmon-epoll because it scales
better.

This patch resets npfd in the case where we switch to fdmon-epoll.
Forgetting to do so results in the following assertion failure:

  util/fdmon-poll.c:65: fdmon_poll_wait: Assertion `npfd == 0' failed.

Fixes: 1f050a46 ("aio-posix: extract ppoll(2) and epoll(7) fd monitoring")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1869952
Message-Id: <20200915120339.702938-2-stefanha@redhat.com>