lib: Lessen restrictions on VIR_DOMAIN_START_AUTODESTROY

lib: Lessen restrictions on VIR_DOMAIN_START_AUTODESTROY

Apart from migrating the VM to a remote host where we can't honour the
VIR_DOMAIN_START_AUTODESTROY flag properly, restricting APIs which just
modify the state of the VM does not make much sense.

Change the wording of the documentation for VIR_DOMAIN_START_AUTODESTROY
so that snapshots and saving to a file may be permitted as they
semantically don't clash with the flag itself. Otherwise we'd have to
forbid other APIs, such as virDomainDestroy as well.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>