Lack of default MTU customisation support for QEMU on Linux

**Describe the bug**
Describe what your problem is.

I can't seem to find a nice way to set the MTU of the guest interface when using QEMU.

I can set the MTU of the default multipass host bridge interface ok using netplan, and the guest network config has to be done via custom cloud-init. This is is a little cludgy. Also, it seems I can't hook into the actual netplan config at all using cloud-init [ it always get's overwritten by the default ], so I'm having to use bootcmd and hard coding the expected interface name. It would be nice if you could jsut tell multipass your desireded default `mtu`, and the host bridge and guest netplan config was created to enforce it automatically.

Current cloud init looks like:
```
#cloud-config

bootcmd:
- [ ip, l, set, ens3, mtu, 1420 ]
```

**To Reproduce**
How, and what happened?
1. Install multipass
2. Use local.driver=qemu
3. default network bridge and all guest interfaces are hard coded to an MTU of `1500`

**Expected behavior**
What did you expect to happen?
Some kind of `multipass set` command to specify a custom default MTU.

**Logs**
Please provide logs from the daemon, see [accessing logs](https://multipass.run/docs/accessing-logs) on where to find them on your platform.

**Additional info**
- Ubuntu 20.04
- 1.12.2
- local.driver=qemu

**Additional context**
Add any other context about the problem here.

Start at the QEMU network setup selected by local.driver=qemu and the multipass set configuration path; compare how the host bridge and guest netplan currently receive the hard-coded MTU of 1500. Done means a custom default MTU can be specified and is applied consistently to the host bridge and guest interfaces without hard-coded cloud-init bootcmd settings.

Written by the indexing model from the issue text.