Increase kvm sonic memory from 3GB to 4GB. (#7253)

What is the motivation for this PR?
Increase KVM sonic memory from 3GiB to 4GiB. Because some cases raised error log created by the tool memory_threshold_check, like free memory xx is less than memory threshold xx. And the threshold is based on total memory.

How did you do it?
Modify memory set in ansible/roles/vm_set/templates/sonic.xml.j2.

Notes: memory tag is the maximum allocation of memory and currentMemory tag is the actual allocation of memory.

How did you verify/test it?
Run TC and no memory_threshold_check issue.

Signed-off-by: Chun'ang Li <chunangli@microsoft.com>

Author: lerry-lee

ansible/roles/vm_set/templates/sonic.xml.j2

@@ -7,8 +7,8 @@
   <memory unit='GB'>8</memory>
   <vcpu placement='static'>16</vcpu>
 {% else %}
-  <memory unit='KiB'>3072000</memory>
-  <currentMemory unit='KiB'>3072000</currentMemory>
+  <memory unit='GiB'>4</memory>
+  <currentMemory unit='GiB'>4</currentMemory>
   <vcpu placement='static'>4</vcpu>
 {% endif %}
   <resource>