[archiso] Allow persistent cow dm-snapshots.
Add some options to control where all COW files will be located. Until this moment all files are located in a tmpfs filesystem. Now is posible to set a device via a filesystem label or device node plus a directory, where all these files will be stored. All dm-snapshot devices will be persistent by default, but this can be changed if wanted. Take care, a filesystem that does not support sparse files maybe is not the best choice for COW files, because they are created with the same size (is apparent) like the read-only device (the image.fs inside .sfs). Of course sooner or later, depending on use, these files actually end up being as big as the read-only device. KNOW-ISSUE: On shutdown in step "Unmounting Filesystems" will [FAIL], all filesystem except $cow_device, will be unmounted/remounted-ro. For this reason this change needs an archiso_shutdown hook, thats unmount all block devices used by archiso. (Work in progress) Signed-off-by: Gerardo Exequiel Pozzi <firstname.lastname@example.org>