backup: add option to on-the-fly renaming conflicting VMs
backup: add option to on-the-fly renaming conflicting VMs

Fixes QubesOS/qubes-issues#869

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