loader/multiboot: fix multiboot loading

The current multiboot loader code doesn't clean the metadata added to the
kernel after the bi_load64 dry run, which breaks accounting of the required
memory for the metadata.

This issue didn't show itself before because all the metadata items where small
(8bytes), but after r316343 there's a big blob in the metadata, which triggers
this. Fix it by cleaning the metadata added to the kernel after the bi_load64
dry run. Also add a comment describing the memory layout when booting using
multiboot (Xen Dom0).

This unbreaks booting a FreeBSD/Xen Dom0 after r316343.

MFC after: 3 weeks
Sponsored by: Citrix Systems R&D