x86: do away with the boot time low-memory 1:1 mapping

x86: do away with the boot time low-memory 1:1 mapping

By doing so, we're no longer restricted to be able to place all boot
loader modules into the low 1Gb/4Gb (32-/64-bit) of memory, nor is
there a dependency anymore on where the boot loader places the
modules.

We're also no longer restricted to copy the modules into a place below
4Gb, nor to put them all together into a single piece of memory.

Further it allows even the 32-bit Dom0 kernel to be loaded anywhere in
physical memory (except if it doesn't support PAE-above-4G).

Signed-off-by: Jan Beulich <jbeulich@novell.com>