Close #453: Support mesa-dri-virtio in Qemu

Created by: ollieparanoid

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).