memory: replace deprecated check_running() call

memory: replace deprecated check_running() call

PVE::QemuServer::check_running() does both
PVE::QemuConfig::assert_config_exists_on_node()
PVE::QemuServer::Helpers::vm_running_locally()

The former one isn't needed here when doing hotplug, because the API
already assert that the VM config exists. It also would introduce a
new cyclic dependency between PVE::QemuServer::Memory <->
PVE::QemuConfig with the proposed virtio-mem patch set.

In preparation to remove the cyclic include of PVE::QemuServer in the
memory module.

No functional change intended.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>