Launch postmarketOS in a qemu virtual machine

In postmarketOS we are now able to generate system images with the correct configuration so that they can boot already using qemu-

This branch implements a new command pmbootstrap qemu to launch postmarketos in a qemu virtual machine. x86_64 and arm architectures are well supported, while aarch64 support depends on the qemu version that you have. i386 support was dropped.

This command is very handy because you don't have to set all the qemu parameters, pmbootstrap does it for you.

In this PR we also made some changes to the qemu-* aports because they didn't work in qemu previously.

Testing and comments are very welcome!

How to test

First build the image you want to test (it's recommended to use --no-fde)

# choose qemu-vexpress for arm and qemu-amd64 for x86
pmbootstrap init
pmbootstrap install

Then run qemu

# Runs qemu in your host architecture
pmbootstrap qemu

# Runs qemu in a different architecture
pmbootstrap qemu --arch arm
pmbootstrap qemu --arch aarch64

Note that the image that is launched depends on the --arch parameter, not the configured one in init.