qemu: Remove the managed state file only if restoring succeeded

qemu: Remove the managed state file only if restoring succeeded

1) Both "qemuDomainStartWithFlags" and "qemuAutostartDomain" try to
restore the domain from managedsave'ed image if it exists (by
invoking "qemuDomainObjRestore"), but it unlinks the image even
if restoring fails, which causes data loss. (This problem exists
for "virsh managedsave dom; virsh start dom").

The fix for is to unlink the managed state file only if restoring
succeeded.

2) For "virsh save dom; virsh restore dom;", it can cause data
corruption if one reuse the saved state file for restoring. Add
doc to tell user about it.

3) In "qemuDomainObjStart", if "managed_save" is NULL, we shouldn't
fallback to start the domain, skipping it to cleanup as a incidental
fix. Discovered by Eric.