vhost-scsi: init backend features earlier

vhost-scsi: init backend features earlier

As vhost core can use backend_features during init, clear it earlier to
avoid using uninitialized memory.
This use would be harmless since vhost scsi ignores the result
anyway, but initializing earlier will help prevent valgrind errors,
and make scsi and net behave similarly.

Cc: qemu-stable@nongnu.org
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 3a1655fc53a2d0375dc0b8cd358405c2cae288e3)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>