qga: vss-win32: Use NULL as an invalid pointer for OpenEvent and CreateEvent

OpenEvent and CreateEvent WinAPI return NULL when failed to open/create
events handles, instead of INVALID_HANDLE_VALUE (although their return
types are HANDLE).
This replaces INVALID_HANDLE_VALUE related to event handles with NULL.

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Reviewed-by: Gal Hammer <ghammer@redhat.com>
Reviewed-by: Yan Vugenfirer <yvugenfi@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
