fix(mmio/persist): do not unwrap error if VirtioMem restore fails

Return the error up the stack instead of unwrapping it.

Note that on PCI this error is unwrapped for every device so there's no
change there.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

src/vmm/src/device_manager/persist.rs

@@ -595,7 +595,7 @@ impl<'a> Persist<'a> for MMIODeviceManager {
 
         if let Some(memory_state) = &state.memory_device {
             let ctor_args = VirtioMemConstructorArgs::new(Arc::clone(vm));
-            let device = VirtioMem::restore(ctor_args, &memory_state.device_state).unwrap();
+            let device = VirtioMem::restore(ctor_args, &memory_state.device_state)?;
 
             constructor_args.vm_resources.memory_hotplug = Some(MemoryHotplugConfig {
                 total_size_mib: device.total_size_mib(),