Restoring a snapshot that uses virtio-mem backed by a file fails

[alex-matei]

Describe the bug

The memory region that is associated with the hotpluggable part of a virtio-mem zone isn't backed by the file specified in the MemoryZoneConfig. The file is used only for the fixed part of the zone. When you try to restore a snapshot with virtio-mem, the backing file is used for all its regions. This results in the following error:

  Error restoring VM: VmRestore(MemoryManager(GuestMemoryRegion(MappingPastEof)))

To Reproduce

First terminal

truncate -s 512M /tmp/mem
cloud-hypervisor --serial tty --console off --kernel /opt/kata/share/kata-containers/vmlinux.container -v --cmdline "console=ttyS0,115200n8" --initramfs /opt/kata/share/kata-containers/kata-alpine-3.15.initrd --api-socket path=/tmp/test.sock --memory size=0,hotplug_method=virtio-mem  --memory-zone id=mem0,size=512M,file=/tmp/mem,shared=on,hotplug_size=1G

Second terminal

ch-remote --api-socket /tmp/test.sock pause
mkdir /tmp/template
ch-remote --api-socket /tmp/test.sock snapshot file:///tmp/template
killall cloud-hypervisor

First terminal

cloud-hypervisor --restore source_url=file:///tmp/template --api-socket path=/tmp/test.sock -v