When running qemu:///system instance, libvirtd runs as root,
but QEMU may optionally be configured to run non-root. When
then saving a guest to a state file, the file is initially
created as root, and thus QEMU cannot write to it. It is also
missing labelling required to allow access via SELinux.

* src/qemu/qemu_driver.c: Set ownership on save image before
  running migrate command in virDomainSave impl. Call out to
  security driver to set save image labelling
* src/security/security_driver.h: Add driver APIs for setting
  and restoring saved state file labelling
* src/security/security_selinux.c: Implement saved state file
  labelling for SELinux