Fix PluginVm unsharing

Explicitly check PluginVm status rather than relying on
CrostiniManager to know the sate of VM.

Bug: b:135772268
Change-Id: I61a02fc405a7dcfa7889c3d3700a3b3c22ad7f6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1677166
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672428}