hw/9pfs: Fix Kconfig dependency problem between 9pfs and Xen

hw/9pfs: Fix Kconfig dependency problem between 9pfs and Xen

Commit b2c00bce54c ("meson: convert hw/9pfs, cleanup") introduced
CONFIG_9PFS (probably a wrong conflict resolution). This config is
not used anywhere. Backends depend on CONFIG_FSDEV_9P which itself
depends on CONFIG_VIRTFS.

Remove the invalid CONFIG_9PFS and use CONFIG_FSDEV_9P instead, to
fix the './configure --without-default-devices --enable-xen' build:

  /usr/bin/ld: libcommon.fa.p/hw_xen_xen-legacy-backend.c.o: in function `xen_be_register_common':
  hw/xen/xen-legacy-backend.c:754: undefined reference to `xen_9pfs_ops'
  /usr/bin/ld: libcommon.fa.p/fsdev_qemu-fsdev.c.o:(.data.rel+0x8): undefined reference to `local_ops'
  /usr/bin/ld: libcommon.fa.p/fsdev_qemu-fsdev.c.o:(.data.rel+0x20): undefined reference to `synth_ops'
  /usr/bin/ld: libcommon.fa.p/fsdev_qemu-fsdev.c.o:(.data.rel+0x38): undefined reference to `proxy_ops'
  collect2: error: ld returned 1 exit status

Fixes: b2c00bce54c ("meson: convert hw/9pfs, cleanup")
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20201104115706.3101190-3-philmd@redhat.com>
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>