virSecurityManagerTransactionCommit: Accept pid == -1

virSecurityManagerTransactionCommit: Accept pid == -1

It will be desirable to run transactions more often than we
currently do. Even if the domain we're relabeling the paths for
does not run in a namespace. If that's the case, there is no need
to fork() as we are already running in the right namespace. To
differentiate whether transaction code should fork() or not the
@pid argument now accepts -1 (which means do not fork).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>