devices: fs: Add Filesystem trait
devices: fs: Add Filesystem trait

Add the `Filesystem` trait, which is the main interface between the
transport and the actual file system implementation.

BUG=b:136128319
TEST=`tast run vm.VirtioFs`

Change-Id: Ic8bc9e231652020501e10ad0be810a9f66e90b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1757241
Auto-Submit: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Commit-Queue: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

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