qemu-ga: Fix null pointer passed to unlink in failure branch

Clang reports this warning:

Null pointer passed as an argument to a 'nonnull' parameter

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>