devices: clarify error on attaching non-persistent device to stopped VM
devices: clarify error on attaching non-persistent device to stopped VM

Fixes QubesOS/qubes-issues#3195

marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin