bhyve: silent destroy command errors on cleanup

bhyve: silent destroy command errors on cleanup

When virBhyveProcessStart() fails, it tries to unload
a guest that could have been already loaded using
bhyveload(8) to make sure not to leave it hanging in memory.

However, we could fail before loading a VM into memory,
so 'bhyvectl --destroy' command will fail and print
an error message that looks confusing to users.

So ignore errors when running this in cleanup.