x86/pvh: copy data from low 1MB to Dom0 physmap instead of mapping it

x86/pvh: copy data from low 1MB to Dom0 physmap instead of mapping it

Identity mapping RAM regions on the low 1MB for Dom0 is not ideal,
since there's data there that could be used by Xen during runtime
(like the AP trampoline), so instead of identity mapping the low 1MB
into the Dom0 physmap populate those RAM regions and copy the data.

Note that this allows to remove unshare_xen_page_with_guest since the
only caller was the PVH Dom0 builder.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>