[Filesystem] Change setVirtualPath method to be not enumerable

[Filesystem] Change setVirtualPath method to be not enumerable

Feature of setVirtualPath is needed for proper handling virtual paths
in thread service model - related change:
https://review.tizen.org/gerrit/gitweb?p=platform/core/api/webapi-plugins.git;a=commit;h=4870490975f9cbfa3eb16cc892f3593cb74b6668

[Verification] Code compiles without errors. Code was properly
formatted.
Checked in chrome console - setVirtualPath is not iterable, but still
accessible.

Change-Id: Ia3d2720cd1267d0a4d002538f319cfd993e83d4a