[vs][multi-asic]: Add new platforms for multi-asic vs. (#8269)

Why I did it
End goal: To have azure pipeline job to run multi-asic VS tests.
Intermediate goal: Require multi-asic KVM image so that the test can be run.
The difference between single asic and multi-asic KVM image is asic.conf file which has different NUM_ASIC values.
Idea behind the approach in this PR to attain the intermediate goal above:

Ease of building multi-asic KVM image so that any user or azure pipeline can use a simple make command to generate single or multi-asic KVM images as required.
Use a single onie installer image and multiple KVM images for single and multi-asic images.
Current scenario:
For VS platform, sonic-vs.bin is generated which is the onie installer image and sonic-vs.img.gz is generated which is the KVM iamge.
Scenario to be achieved:
sonic-vs.bin - which will include single asic platform, 4 asic platform and 6 asic platform.
sonic-vs.img.gz - single asic KVM image
sonic-4asic-vs.img.gz - 4 asic KVM image
sonic-6asic-vs.img.gz - 6 asic KVM image

In this PR, 2 new platforms are added for 4-asic and 6-asic VS.

How I did it
Create 4-asic and 6-asic device directories with the required files and hwsku files.
Add onie-recovery image information in vs platform.
How to verify it
After this PR change, no build change.
sonic-vs.bin onie installer image should include information of new multi-asic vs platforms.