Retry allocation failure in grub_relocator32_boot.

The small amount of memory allocated in grub_relocator32_boot for
the temporary GDT is taken out of the low 64K of guest RAM. Since
this happens at the end of grub's run just prior to exit, it's
possible that all of this memory has been allocated by other
subsystems (the multiboot loader can this).

Fix by retrying the allocation at a location directly after the last
existing allocation. A utility routine, grub_relocator_end(), will return
this guest address which is then rounded up to the required
alignment and used for a direct allocation call.
Multiboot fixes.

- in grub_multiboot_add_module(), fix NULL deref of modules_last when
  adding an element to the list. Most likely fine on real h/w, but
  not in user-space grub-emu where it results in a segv.

- comment out video parameter retrieval. This doesn't work for
  serial-port-only grub-bhyve and results in errors being
  returned that prevent booting.
Remove harmless-but-disconcerting warning text.

The multiboot loader prints out a large warning if no video output
was found. This is a non-issue with grub-bhvye since there is only
serial-console output.
Enable building the multiboot loader with the grub-emu target.