exexdriver: Make Command.GetExitCode an internal call

This code only works for backends that directly spawn the child
via the Command. It will not work for the libvirt backend. So
we move this code into the individual backends that need it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3805 from alexlarsson/execdriver-drop-getexitcode

exexdriver: Make Command.GetExitCode an internal call