xen: find unused contiguous memory area

xen: find unused contiguous memory area

For being able to relocate pre-allocated data areas like initrd or
p2m list it is mandatory to find a contiguous memory area which is
not yet in use and doesn't conflict with the memory map we want to
be in effect.

In case such an area is found reserve it at once as this will be
required to be done in any case.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 9ddac5b724a9465e27f25a0aa943e92c8341a85b)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>