x86/boot: Drop incorrect mapping at l2_xenmap[0]

x86/boot: Drop incorrect mapping at l2_xenmap[0]

It has been 4 years since the default load address changed from 1M to 2M, and
_stext ceased residing in l2_xenmap[0].  We should not be inserting an unused
mapping.

To ensure we don't create mappings accidentally, loop from 0 and obey
_PAGE_PRESENT on all entries.

Fixes: 7ed93f3a0dff ("x86: change default load address from 1 MiB to 2 MiB")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>