Add new test case: boot guest with snapshot=on and data-plane enable

Signed-off-by: NAihua Liang <aliang@redhat.com>

qemu/tests/cfg/data_plane_snapshot.cfg

+- data_plane_snapshot:
+    virt_test_type = qemu
+    type = boot
+    image_snapshot = yes
+    only Host_RHEL.m7
+    only virtio_blk virtio_scsi
+    iothreads = iothread0
+    virtio_blk:
+        blk_extra_params_image1 = "iothread=${iothreads}"
+    virtio_scsi:
+        no Host_RHEL.m7.u0, Host_RHEL.m7.u1, Host_RHEL.m7.u2
+        bus_extra_params_image1 = "iothread=${iothreads}"