feat(vm_provision_mixin): set agent=1 on VM create for guest-agent channel

Proxmox only opens the virtio-serial channel qemu-guest-agent needs when
agent=1 is set at VM creation — without it, the agent package can be
installed but never actually reachable.

napalm_proxmox/vm_provision_mixin.py

@@ -273,6 +273,10 @@ class ProxmoxVMProvisionMixin:
                 cores=cpu,
                 ostype="l26",
                 scsihw="virtio-scsi-pci",
+                # Without this, Proxmox never attaches the virtio-serial
+                # channel the QEMU guest agent needs — get_vm_status's
+                # agent queries (below) would have nothing to talk to.
+                agent="1",
             )
 
             # Step 3: Download cloud image (cached) and import as root disk