virStorageBackendCreateRaw: createRawFile already reported the
exact error.

Before the fix:

error: Failed to create vol vol-create.img
error: cannot create path '/var/lib/libvirt/images/vol-create.img': Unknown error 18446744073709551597

After the fix:

error: Failed to create vol vol-create.img
error: cannot fill file '/var/lib/libvirt/images/vol-create.img': No space left on device