Makefile: Let the 'help' target list the helper targets

Makefile: Let the 'help' target list the helper targets

List the name of the helper targets when calling 'make help',
along with the tool targets:

  $ make help
  [...]

  Helper targets:
    fsdev/virtfs-proxy-helper      - Build virtfs-proxy-helper
    scsi/qemu-pr-helper            - Build qemu-pr-helper
    qemu-bridge-helper             - Build qemu-bridge-helper
    vhost-user-gpu                 - Build vhost-user-gpu
    virtiofsd                      - Build virtiofsd

  Tools targets:
    qemu-ga                        - Build qemu-ga tool
    qemu-keymap                    - Build qemu-keymap tool
    elf2dmp                        - Build elf2dmp tool
    ivshmem-client                 - Build ivshmem-client tool
    ivshmem-server                 - Build ivshmem-server tool
    qemu-nbd                       - Build qemu-nbd tool
    qemu-storage-daemon            - Build qemu-storage-daemon tool
    qemu-img                       - Build qemu-img tool
    qemu-io                        - Build qemu-io tool
    qemu-edid                      - Build qemu-edid tool

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>