sys_util: add method for copying PollEvents
sys_util: add method for copying PollEvents

Making a copy of PollEvents is useful to drop the PollEvents structure
which borrows from a PollContext. Even though immutably borrowing from a
PollContext does not prevent any operations on a PollContext, it does
prevent mutable method calls on any structure that owns PollContext.

TEST=None
BUG=chromium:816692

Change-Id: I9527fd5c122a703933deb973ad549b792226e4c6
Reviewed-on: https://chromium-review.googlesource.com/1000101
Commit-Ready: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm