vhost-user-fs: add the "bootindex" property

vhost-user-fs: add the "bootindex" property

virtio-fs qualifies as a bootable device minimally under OVMF, but
currently the necessary "bootindex" property is missing. Add the property.

Expose the property only in the PCI device, for now. There is no boot
support for virtiofs on s390x (ccw) for the time being [1] [2], so leave
the CCW device unchanged. Add the property to the base device still,
because adding the alias to the CCW device later will be easier this way
[3].

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg01745.html
[2] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg01870.html
[3] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg01751.html

Example OpenFirmware device path for the "vhost-user-fs-pci" device in the
"bootorder" fw_cfg file:

  /pci@i0cf8/pci-bridge@1,6/pci1af4,105a@0/filesystem@0

Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Ján Tomko <jtomko@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: virtio-fs@redhat.com
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210112131603.12686-1-lersek@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>