efi_loader: check CreateEvent() parameters

Add some of the invalid parameter checks described in the UEFI
specification for CreateEvent().  This does not include checking
the validity of the type and tpl parameters.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Acked-By: Heinrich Schuchardt <xypron.glpk@gmx.de>
[agraf: fix checkpatch.pl indent warning]
Signed-off-by: Alexander Graf <agraf@suse.de>