poll-loop: Track the poll timeout as an absolute, not a relative time.

poll-loop: Track the poll timeout as an absolute, not a relative time.

This is a necessary prerequisite for allowing time to be "fast forwarded"
in unit tests, to keep tests that depend on the passage of time from
running in real time.  Without this change, a code sequence like this:

	poll_timer_wait(1000);
	...fast forward time 5 seconds...
	poll_block();

would still sleep for a second, because the poll_loop module would still
have a relative timeout of 1000 ms.

Signed-off-by: Ben Pfaff <blp@nicira.com>