virCommand: Introduce APIs for core scheduling

virCommand: Introduce APIs for core scheduling

There are two modes of core scheduling that are handy wrt
virCommand:

1) create new trusted group when executing a virCommand

2) place freshly executed virCommand into the trusted group of
   another process.

Therefore, implement these two new operations as new APIs:
virCommandSetRunAlone() and virCommandSetRunAmong(),
respectively.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>