[build] Increase vs platform kvm disk size (#10001)

#### Why I did it
Info: Attempting file://dev/vdb/onie-installer ...
Info: Attempting file://dev/vdb/onie-installer.bin ...
cp: write error: No space left on device
Failure: local_fs_run():/dev/vdb Unable to copy /tmp/tmp.CPY0ad/onie-installer.bin to tmpfs

vs image is failing. Increase kvm device space.

scripts/build_kvm_image.sh

@@ -4,7 +4,7 @@
 #
 #  SPDX-License-Identifier:     GPL-2.0
 
-MEM=4096
+MEM=8192
 DISK=$1
 ONIE_RECOVERY_ISO=$2
 INSTALLER=$3