Add removable-media permission

[thatLeaflet]

As this snap does not use the file picker portal (issue #22), you can only create ISOs that are stored in your home directory.

However, some people, such as myself, use Ventoy for booting ISOs. I often want to use the ISOs stored there to create a new VM. Currently, if I want to use one of these ISOs in this snap, I first need to copy it into my home directory, which is inconvenient.