1. Output of `losetup` command contains `\n` - strip it.
2. Provide read-only option - if device info hasn't propagated to qubesd
yet, it will not be set automatically.
FixesQubesOS/qubes-issues#3146
Booting a VM from cdrom require attaching the device before VM startup,
which is possible only in persistent mode. But for qvm-start --cdrom
adding a cdrom only temporarily, use new update_persistence() function
to convert the assignment to temporary one.
FixesQubesOS/qubes-issues#3055
