xen: add capability to load initrd outside of initial mapping

xen: add capability to load initrd outside of initial mapping

Modern pvops linux kernels support an initrd not covered by the initial
mapping. This capability is flagged by an elf-note.

In case the elf-note is set by the kernel don't place the initrd into
the initial mapping. This will allow to load larger initrds and/or
support domains with larger memory, as the initial mapping is limited
to 2GB and it is containing the p2m list.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>