Clarify BUGS wrt. reserving memory for the hypervisor

Clarify BUGS wrt. reserving memory for the hypervisor

The previous wording might be understood as "leave memory unused in the
primary domain", which is precisely what causes the hypervisor to reject
the configuration since ldomctl(8) would implicitly allocate all remaining
memory for the primary domain.

Make sure that primary domain memory should be assigned explicitly so the
total amount of allocated memory is less than physically available, i.e.
the hypervisor will have even more memory available and configurations can
boot again.

OK stsp