refactor: split check_running into _exists_ and _running_

refactor: split check_running into _exists_ and _running_

vm_exists_on_node in PVE::QemuConfig checks if a config file for a vmid
exists

vm_running_locally in PVE::QemuServer::Helpers checks if a VM is running
on the local machine by probing its pidfile and checking /proc/.../cmdline

check_running is left in QemuServer for compatibility, but changed to
simply call the two new helper functions.

Both methods are also correctly mocked for testing snapshots.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>