Unable to boot disks from read only file systems

[MagnusS]

When booting an iso from a read-only nix location, qemu fails with:

<vm> qemu-system-x86_64: Could not open 'grub.iso': Read-only file system

The iso is attached using the parameter -drive file=grub.iso,format=raw,if=ide,media=disk

Changing media manually to media=cdrom boots the image.

[MagnusS]

Currently it's not possible to specify other media-types with vm.boot(): https://github.com/includeos/vmrunner/blob/master/vmrunner/vmrunner.py#L1027

And only disk is supported in the schema: https://github.com/includeos/vmrunner/blob/master/vmrunner/vm.schema.json#L59

[MagnusS]

This fix is needed to run the IncludeOS grub test with a read only file system (e.g. via nix' testrunner which uses nixos)