xen/arm: domain_build: Rework initrd_load to use the generic copy helper

xen/arm: domain_build: Rework initrd_load to use the generic copy helper

The function initrd_load is dealing with IPA but uses gvirt_to_maddr to
do the translation. This is currently working fine because the stage-1 MMU
is disabled.

Furthermore, the function is implementing its own copy to guest resulting
in code duplication and making more difficult to update the logic in
page-tables (such support for Populate On Demand).

The new copy_to_guest_phys_flush_dcache could be used here by temporarily
mapping the full initrd in the virtual space.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>