Due to how a Proxmox cluster reacts to a VM shutdown command when
running in HA mode, the VM might still be running when the shutdown API
calls returns. This commit adds a loop that actively waits for the VM's
status to change to "stopped" (while also accounting for the shutdown
timeout) after the call's return.

…te function

…cluster

This commit forces the plugin to wait for a VM to actually run after
requesting it to be started. This avoids problems with Proxmox's High
Availability mode, where a start request may not be immediately honoured
by the cluster.