vfio/pci: Fix buffer overrun when writing the VF token

qemu_uuid_unparse() includes a trailing NUL when writing the uuid
string and the buffer size should be UUID_FMT_LEN + 1 bytes. Use the
recently added UUID_STR_LEN which defines the correct size.

Fixes: CID 1522913
Fixes: 2dca1b37 ("vfio/pci: add support for VF token")
Cc: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: "Denis V. Lunev" <den@openvz.org>
Signed-off-by: Cédric Le Goater <clg@redhat.com>