EFI: add architecture functions for pre/post ExitBootServices

EFI: add architecture functions for pre/post ExitBootServices

The UEFI ExitBootServices function is invoked to transition the
system to the 'runtime' mode of operation, and is done right before
transitioning from the EFI loader code into Xen proper. x86 does some
arch specific memory management (trampoline) before exit boot services,
and the code that transitions from the EFI application state to Xen
is architecture specific.  This patch adds two functions, one pre
and one post ExitBootServices to allow each architecture to
to handle these cases in a customized manner.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>