Use virtio-scsi to attach the image. virtio-blk (if=virtio) is inconsistent about the used media type.

ISO testing doesn't require the emulated firmware's PXE stack.
For UEFI, it only disable iPXE in favor of EDK II network stack. https://github.com/tianocore/tianocore.github.io/wiki/EDKII-Network-Over-QEMU#verification-result

Various bash script improvements.

scripts/run_archiso.sh:
A script to run a built archiso image using qemu.
It can run the image on an emulated BIOS (using seabios) or UEFI (using edk2-ovmf) system.

.gitlab-ci.yml:
Adding scripts/run_archiso.sh to shellcheck call.

Closes #28