kvm: require JOIN_MEMORY_REGIONS_WORKS

kvm: require JOIN_MEMORY_REGIONS_WORKS

We already require DESTROY_MEMORY_REGION_WORKS, JOIN_MEMORY_REGIONS_WORKS
was added just half a year later.

In addition, with flatview overlapping memory regions are first
removed before adding the changed one. So we can't really detect joining
memory regions this way.

Let's just get rid of this special handling.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170911174933.20789-2-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>