x86/boot: Allocate one extra module slot for Xen image placement

x86/boot: Allocate one extra module slot for Xen image placement

Commit 9589927 (x86/mb2: avoid Xen image when looking for
module/crashkernel position) fixed relocation issues for
Multiboot2 protocol. Unfortunately it missed to allocate
module slot for Xen image placement in early boot path.
So, let's fix it right now.

Reported-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 4c5f9dbebc0bd2afee1ecd936c74ffe65756950f
master date: 2018-09-27 11:17:47 +0100