EFI: suppress bogus loader warning

This was accidentally lost in commit fbc3d9a220 ("EFI: add
efi_arch_handle_cmdline() for processing commandline"), leading to the
"Unknown command line option" warning being printed whenever options
get passed to the core hypervisor or the Dom0 kernel.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -311,6 +311,7 @@ static unsigned int __init get_argv(unsi
                 ++argc;
             else if ( prev && wstrcmp(prev, L"--") == 0 )
             {
+                --argv;
                 if ( options )
                     *options = cmdline;
                 break;