Don't take snapshots while the VM is writing critical system files. Use qemu-img snapshot -l to list, -a to revert.
There are a few websites that offer free and public domain Windows XP Qcow2 files for download. Some popular options include: Windows Xp Qcow2 File Download
Before you search for a "Windows XP Qcow2 file download," you must understand the licensing. Microsoft no longer supports Windows XP (since April 8, 2014), but Don't take snapshots while the VM is writing