xen: arm: correct return value of raw_copy_{to/from}_guest_*, raw_clear_guest

xen: arm: correct return value of raw_copy_{to/from}_guest_*, raw_clear_guest

This is a generic interface which is supposed to return the number of bytes
which were not copied. Make it so.

Update the incorrect callers prepare_dtb, decode_thumb{2} and
xenmem_add_to_physmap_range.

In the xenmem_add_to_physmap_range case, observe that we are not propagating
errors from xenmem_add_to_physmap_one and do so.

In the decode_thumb case and an emacs magic block to decode.c

Make the flush_dcache parameter to the helper an int while at it.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>