cgroup: Drop resource partition from virSystemdMakeScopeName

cgroup: Drop resource partition from virSystemdMakeScopeName

The scope name, even according to our docs is
"machine-$DRIVER\x2d$VMNAME.scope" virSystemdMakeScopeName would use the
resource partition name instead of "machine-" if it was specified thus
creating invalid scope paths.

This makes libvirt drop cgroups for a VM that uses custom resource
partition upon reconnecting since the detected scope name would not
match the expected name generated by virSystemdMakeScopeName.

The error is exposed by the following log entry:

debug : virCgroupValidateMachineGroup:302 : Name 'machine-qemu\x2dtestvm.scope' for controller 'cpu' does not match 'testvm', 'testvm.libvirt-qemu' or 'machine-test-qemu\x2dtestvm.scope'

for a "/machine/test" resource and "testvm" vm.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1238570