Factor out copying kvm_memslots from allocating the memory for new ones
in preparation for adding a new lock to protect the arch-specific fields
of the memslots.

No functional change intended.
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NBen Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-4-bgardon@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>