crosvm: main - Allow read/write block devices
crosvm: main - Allow read/write block devices

Add an option for allowing a writable disk.
Allow specifying multiple disks.
Don't assume the path to the root device, force the user to specify a
correct command line option.

Change-Id: I87dea1152e1f81c0cde45ad47e2c4cf0f187918e
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/569450
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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