x86: allow passing initrd to kernel without exposing it through the initial mapping

x86: allow passing initrd to kernel without exposing it through the initial mapping

The (Dom0 only for now) kernel can indicate that it doesn't need its
initrd mapped through a newly added ELF note - it gets passed the PFN
of the initrd in this case instead of the virtual address.

Even for kernels not making use of the new feature, the initrd will
no longer get copied into the initial mapping, but the memory it lives
in will get assigned to and mapped for the guest instead.

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