crosvm: block - Fill upper 4 bytes of disk size
crosvm: block - Fill upper 4 bytes of disk size

Fill the upper for bytes of the disk size configuration.  The size is a
64bit value accessed with two 32bit reads.

The guest is permitted to read at any byte offset in the config space.
Allow it to do so, even if it doesn't make much sense.

Change-Id: I5d02620a8751b31784e419ae6a57173a2e212b8f
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/569359
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

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