sys_util: shm: make using names with shared memory more convenient
sys_util: shm: make using names with shared memory more convenient

This change adds a string based constructor of `SharedMemory` as well as
adding a method for retrieving that name from the underlying file. This
change also includes a new anonymous constructor.

TEST=cargo test -p sys_util
BUG=None

Change-Id: Ibd7a28851c8a0f41e595ee35b35f0d06fef1e1d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1797187
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Commit-Queue: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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