Restore from NAS backup is not failing for VM with template / disks < 1 GB

[sureshanaparti]

problem

Restore from NAS backup is not failing for VM with template / disks < 1 GB, fails with "Root disk size should be a positive number." error.

It seems regression from create VM from backup feature (#10140)

The size computation below, returns 0 for disks < 1 GB.

cloudstack/server/src/main/java/org/apache/cloudstack/backup/BackupManagerImpl.java

Lines 1168 to 1169 in e78b5cc

	Long size = volume.getSize() / (1024 * 1024 * 1024);
	rootDiskOffering = new VmDiskInfo(diskOffering, size, volume.getMinIops(), volume.getMaxIops());

cloudstack/server/src/main/java/org/apache/cloudstack/backup/BackupManagerImpl.java

Lines 1192 to 1193 in e78b5cc

	Long size = volume.getSize() / (1024 * 1024 * 1024);
	vmDiskInfoList.add(new VmDiskInfo(diskOffering, size, volume.getMinIops(), volume.getMaxIops(), volume.getDeviceId()));

versions

ACS 4.21.0, KVM Hypervisor, NAS repo configured

The steps to reproduce the bug

1. Register a template (< 1 GB) - http://dl.openvm.eu/cloudstack/macchinina/x86_64/macchinina-kvm.qcow2.bz2
2. Deploy a VM using the above template
3. Assign backup offering and take backup
4. Go to the backup ,and restore from it.

What to do about it?

Support Restore from NAS backup for VM with template / disks < 1 GB, document it if unsupported.