storage: extract single volume clone into clone_volume
storage: extract single volume clone into clone_volume

This will be useful for admin.vm.volume.Clone implementation.

QubesOS/qubes-issues#2256

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