Fix: Protect FileProvider.files to avoid concurrent modification
Fix: Protect FileProvider.files to avoid concurrent modification

vim25.debug.FileProvider.files is a shared resource. It should be
protected to avoid concurrent modifications. This change ensure
safe access to the variable by using a mutex.

Closes: #2443

vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi