dom0+vm/qvm-usb: don't destroy xenstore in backend on install
dom0+vm/qvm-usb: don't destroy xenstore in backend on install

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