The mesa driver, which ends up in the installation image, needs to be known
before the installation is done (in other words: when running the qemu action,
it is to late as the image has already been generated). That's why one can
choose the Qemu mesa driver in `pmbootstrap init` now:
```
Device [qemu-amd64]:
Which mesa driver do you prefer for your Qemu device? Only select something other
than the default if you are having graphical problems (such as glitches).
Mesa driver (dri-swrast/dri-virtio) [dri-virtio]:
```
It is still possible to select `dri-swrast`, because `dri-virtio` may not work
in all cases, and that way we could easily debug it or experiment with other
mesa drivers (e.g. the "vmware" one, which is supported by mesa and Qemu).
Other changes:
* `pmbootstrap qemu` accepts a `--display` variable now, which passes the value
directly to `qemu`'s `display` option. It defaults to `sdl,gl=on` (@PureTryOut
reported that to work best with plasma mobile on his PC). `--display` and
`--spice` (which is still working) are mutually exclusive.
* Removed obsolete telnet port pass-through: We only use the debug telnet port
since osk-sdl has been merged.
* Add show-cursor to the Qemu command line, so it shows a cursor in X11
* Refactored the spice code (`command_spice` only returns the spice command,
because it has all necessary information already) and the spice port can be
specified on the commandline now (previously it was hardcoded in one place and
then always looked up from there).
* Start comments with capital letters.
* Keep the log on the screen a bit shorter (e.g. Qemu command is written to the
"pmbootstrap log" anyway, so there's no need to display it again).
* linux-postmarketos-stable: Adjust kernel configs
x86_64, armhf: enable as modules:
CONFIG_DRM_VIRTIO_GPU, CONFIG_VIRTIO_PCI, CONFIG_VIRTIO_BALLOON
aarch64: all 3 options were already enabled as built-in (no change)
* Set '-vga virtio' for mesa-dri-virtio
