devices: virtio: vsock: Use async locks in async contexts

In the vsock code, we use a mutex to protect our connections `HashMap`
so that we can have access to them from multiple async tasks. We were
using a regular synchronous mutex for this, which could cause the async
tasks to block on these mutexes, possibly leading to blocking up the
executor and deadlocking vsock.

We haven't observed any bugs or deadlocks that are directly attributable
to this; it's likely that we are managing to avoid this by not holding
the lock over await points. We should still fix this for correctness, as
we have no way to otherwise enforce that future changes should uphold
the current guarantees.

BUG: b:247548758
TEST: Built and ran crosvm downstream.
Change-Id: I8928514be491f111887fbf1adac7a3f8b38219dd
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4062047
Commit-Queue: Richard Otap <rotap@google.com>
Reviewed-by: Noah Gold <nkgold@google.com>