devices: block: define features as shift counts
devices: block: define features as shift counts

Convert the definition of VIRTIO_BLK_F_FLUSH to a shift count instead of
a bitmask.  This matches the way the features are defined in the virtio
spec and makes block consistent with other device models, such as p9.

BUG=chromium:872973
TEST=cargo test -p devices

Change-Id: Iece974c6f4d826b7bb76622973f08469a7936234
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1170303
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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