Allow to create instances stopped

**What are you trying to do?**

I'm trying to change VM's settings using `modifyvm`, and to do that I have to stop it first.

**What's your proposed solution?**

Actually, I'd like to have an option to set `host-only` network, but it's also okay if I would be able to start VM in a stopped state, change VM settings using `modifyvm`, and start it using as usual using `multipass launch` command

**Additional context**

Workflow I'm following

```bash
cat > cloud-init.yaml <<'EOF'
write_files:
- path: /etc/netplan/60-host-only.yaml
permissions: '0644'
content: |
network:
ethernets:
IFACE_NAME:
optional: yes
dhcp4: yes
dhcp4-overrides:
route-metric: 110
set-name: IFACE_NAME
dhcp6: no
version: 2
runcmd:
- 'sed -i "s/IFACE_NAME/enp0s8/g" /etc/netplan/60-host-only.yaml'
- 'netplan generate'
- 'netplan apply'
EOF
multipass launch docker --cloud-init cloud-init.yaml
multipass stop docker
gsudo -s "%VBOX_MSI_INSTALL_PATH%\vboxmanage" modifyvm docker --nic2 hostonly --hostonlyadapter2 "VirtualBox Host-Only Ethernet Adapter"
multipass start docker
```

Trace the `multipass launch`, `stop`, and `start` entry points and the platform path behind the shown `modifyvm` workflow. Compare the two requested outcomes—creating an initially stopped instance or configuring a host-only network—and define completion as one supported workflow that preserves normal instance startup behavior.

Written by the indexing model from the issue text.