efi: use directmap to access runtime services table

efi: use directmap to access runtime services table

Do not require switching page tables to access (static) information in
the runtime services table itself, use directmap for this. This allows
exiting early from XEN_EFI_query_capsule_capabilities,
XEN_EFI_update_capsule and XEN_EFI_query_variable_info (in case of not
supported call) without all the impact of page table switch.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>