snapshot: fine-tune qemu snapshot revert states

snapshot: fine-tune qemu snapshot revert states

For a system checkpoint of a running or paused domain, it's fairly
easy to honor new flags for altering which state to use after the
revert.  For an inactive snapshot, the revert has to be done while
there is no qemu process, so do back-to-back transitions; this also
lets us revert to inactive snapshots even for transient domains.

* src/qemu/qemu_driver.c (qemuDomainRevertToSnapshot): Support new
flags.