efi_loader: allowable event types in CreateEventEx()

CreateEventEx() does not allow the following event types:

* EVT_SIGNAL_EXIT_BOOT_SERVICES
* EVT_SIGNAL_VIRTUAL_ADDRESS_CHANGE

This check is needed to pass the UEFI SCT conformance test.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@csgraf.de>