tools/dombuilder: Remove clear_page() from xc_dom_boot.c

tools/dombuilder: Remove clear_page() from xc_dom_boot.c

pfn 0 is a legitimate (albeit unlikely) frame to use, so skipping it is wrong.
This behaviour appears to exists simply to cover the fact that zero is the
default value of an uninitialised field in dom.

ARM already clears the frames at the point that the pfns are allocated,
meaning that the added clear_page() is wasteful.  Alter x86 to match ARM and
clear the page when it is allocated.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>