snapshot: enforce REVERT_FORCE on qemu

snapshot: enforce REVERT_FORCE on qemu

Implements the documentation for snapshot revert vs. force.

Part of the patch tightens existing behavior (previously, reverting
to an old snapshot without <domain> was blindly attempted, now it
requires force), while part of it relaxes behavior (previously, it
was not possible to revert an active domain to an ABI-incompatible
active snapshot, now force allows this transition).

* src/qemu/qemu_driver.c (qemuDomainRevertToSnapshot): Check for
risky situations, and allow force to get past them.