move kvm files into kvm/
After the reshuffling, update MAINTAINERS accordingly.
Make use of the new directory:

target/s390x/kvm/

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Signed-off-by: Cho, Yu-Chen <acho@suse.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210707105324.23400-14-acho@suse.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>