disk: do not require DiskFile in ToAsyncDisk
disk: do not require DiskFile in ToAsyncDisk

This would complicate the implementation of ToAsyncDisk for qcow and
other formats. Change the trait bounds so that only the minimal set of
traits used in the asynchronous block device is required.

BUG=b:219595052
TEST=cargo build

Change-Id: Id5ece59596b0a0989f953854a9cf4c30d32f7e34
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3824064
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Tested-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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