fish: Add -N option for making prepared disk images.

fish: Add -N option for making prepared disk images.

Previously you might have typed:

$ guestfish
><fs> alloc test1.img 100M
><fs> run
><fs> part-disk /dev/sda mbr
><fs> mkfs ext4 /dev/sda1

now you can do the same with:

$ guestfish -N fs:ext4

Some tests have also been updated to use this new
functionality.