hw/nvram: Always register FW_CFG_DATA_GENERATOR_INTERFACE

hw/nvram: Always register FW_CFG_DATA_GENERATOR_INTERFACE

While the FW_CFG_DATA_GENERATOR_INTERFACE is only consumed
by a device only available using system-mode (fw_cfg), it is
implemented by a crypto component (tls-cipher-suites) which
is always available when crypto is used.

Commit 69699f3055 introduced the following error in the
qemu-storage-daemon binary:

  $ echo -e \
    '{"execute": "qmp_capabilities"}\r\n{"execute": "qom-list-types"}\r\n{"execute": "quit"}\r\n' \
    | storage-daemon/qemu-storage-daemon --chardev stdio,id=qmp0  --monitor qmp0
  {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 5}, "package": ""}, "capabilities": ["oob"]}}
  {"return": {}}
  missing interface 'fw_cfg-data-generator' for object 'tls-creds'
  Aborted (core dumped)

Since QOM dependencies are resolved at runtime, this issue
could not be triggered at linktime, and we don't have test
running the qemu-storage-daemon binary.

Fix by always registering the QOM interface.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Fixes: 69699f3055 ("crypto/tls-cipher-suites: Produce fw_cfg consumable blob")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201006111909.2302081-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>