x86/EFI: fix boot for pre-UEFI systems

x86/EFI: fix boot for pre-UEFI systems

efi/boot.c makes a call to QueryVariableInfo on all systems.  However,
as it is only promised for UEFI 2.0+ systems, pre-UEFI systems can
hang or crash on this call.  The below patch adds a version check, a
technique used in other parts of the Xen EFI codebase.

Signed-off-by: Eric Shelton <eshelton@pobox.com>
Check runtime services version instead of EFI version (while generally
they would be in sync, nothing requires them to be).

Signed-off-by: Jan Beulich <jbeulich@suse.com>