dump: allow target to set the physical base

crash assumes the physical base in the kdump subheader of
makedumpfile formatted dumps is correct. Zero is not correct
for all architectures, so allow it to be changed.

(No functional change.)

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1452542185-10914-5-git-send-email-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
