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: NPaolo Bonzini <pbonzini@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 3a1655fc)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>