Fixes:

 $ ../configure --disable-tools --disable-user
 ../tests/qemu-iotests/meson.build:7:0: ERROR: Unknown variable "qemu_block_tools".

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200821150556.1235625-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>