lxc: Don't crash by forgetting to ref transient domains

lxc: Don't crash by forgetting to ref transient domains

So commit 306b3a8504 tried mimicking behaviour of commit 540c339a25, but
added a virObjectRef(vm) only after virDomainObjListAdd() in
lxcDomainDefineXMLFlags() and not in lxcDomainCreateXMLWithFiles().
That way undefining a domain that was started with different XML than
defined will leave the domain object in a state with not enough
references to then remove it.  Hence any lxcDomainDestroyFlags() called
afterwards crashes the daemon.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1351057

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>