[multi-asic]: Add build param to build multi-asic KVM image. (#11453)

Why I did it
[multi-asic]: Add build param to build multi-asic KVM image.
Require sonic-4asic-vs.img.gz to run multi-asic tests.
BUILD_MULTIASIC_KVM=y has to be added to build parameter to generate multi-asic KVM image which can be eventually used in multi-asic VS testbed.
This is a pipeline that we currently have to generate multi-asic KVM image:
https://github.com/Azure/sonic-buildimage/blob/master/.azure-pipelines/official-build-multi-asic.yml

How I did it
Add BUILD_MULTIASIC_KVM=y to build parameter in pipeline script to generate multi-asic KVM image.

How to verify it
The build param is already used in official-build-multi-asic.yml pipeline.

azure-pipelines.yml

@@ -53,7 +53,7 @@ stages:
   jobs:
   - template: .azure-pipelines/azure-pipelines-build.yml
     parameters:
-      buildOptions: 'USERNAME=admin SONIC_BUILD_JOBS=$(nproc) ${{ variables.VERSION_CONTROL_OPTIONS }}'
+      buildOptions: 'USERNAME=admin SONIC_BUILD_JOBS=$(nproc) BUILD_MULTIASIC_KVM=y ${{ variables.VERSION_CONTROL_OPTIONS }}'
       jobGroups:
       - name: vs