crosvm: add kernel command line builder
crosvm: add kernel command line builder

The kernel command line is dynamically built up based on what devices
are added and what extra parameters are passed in my the user. The
module this CL adds ensures the kernel command line that is built is a
valid one.

TEST=None
BUG=None

Change-Id: Ia2b7b9fae7eb29140ae25ab34119d050984f8ef2
Reviewed-on: https://chromium-review.googlesource.com/514689
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