Although reverting to a snapshot is a form of data loss, this is
normally expected.  However, there are two cases where additional
surprises (failure to run the reverted state, or a break in
connectivity to the domain) can come into play.  Requiring extra
acknowledgment in these cases will make it less likely that
someone can get into an unrecoverable state due to a default revert.

Also create a new error code, so users can distinguish when forcing
would make a difference, rather than having to blindly request force.

* include/libvirt/libvirt.h.in (VIR_DOMAIN_SNAPSHOT_REVERT_FORCE):
New flag.
* src/libvirt.c (virDomainRevertToSnapshot): Document it.
* include/libvirt/virterror.h (VIR_ERR_SNAPSHOT_REVERT_RISKY): New
error value.
* src/util/virterror.c (virErrorMsg): Implement it.
* tools/virsh.c (cmdDomainSnapshotRevert): Add --force to virsh.
* tools/virsh.pod (snapshot-revert): Document it.