kvm: split too big memory section on several memslots

kvm: split too big memory section on several memslots

Max memslot size supported by kvm on s390 is 8Tb,
move logic of splitting RAM in chunks upto 8T to KVM code.

This way it will hide KVM specific restrictions in KVM code
and won't affect board level design decisions. Which would allow
us to avoid misusing memory_region_allocate_system_memory() API
and eventually use a single hostmem backend for guest RAM.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190924144751.24149-4-imammedo@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>