main: add --rwroot option to run
main: add --rwroot option to run

This allows specifying a read-write rootfs (rather than read-only as
with --root), including the automatic kernel command line additions
normally added by --root.

BUG=None
TEST=Boot crosvm with --rwroot and write to root filesystem

Change-Id: I6a3dc9176bffdefe664139cb33bf3e65a751dbf2
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1679531
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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