io_jail: add bind mount
io_jail: add bind mount

This will be used by some device jails to get access to unix sockets
after being jailed.

TEST=None
BUG=None

Change-Id: I870bfb155b275769ccb3248b1441e7c0b8f20ad7
Reviewed-on: https://chromium-review.googlesource.com/558447
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