[docker-vs] Install dependencies for testing DPB (sonic-net#4960)

Added xmltodict and jsondiff packages needed to run vs test cases successfully for DPB.
sonic-utilities PR #766 has a dependency on these packages.

Signed-off-by: Sangita Maity <sangitamaity0211@gmail.com>

platform/vs/docker-sonic-vs/Dockerfile.j2

@@ -57,6 +57,10 @@ RUN pip install urllib3
 RUN pip install requests
 RUN pip install crontab
 
+# Install dependencies for Dynamic Port Breakout
+RUN pip install xmltodict==0.12.0
+RUN pip install jsondiff==1.2.0
+
 {% if docker_sonic_vs_debs.strip() -%}
 # Copy locally-built Debian package dependencies
 {%- for deb in docker_sonic_vs_debs.split(' ') %}