prepare-volatile-img.sh: create volatile.img group accessible
prepare-volatile-img.sh: create volatile.img group accessible

Otherwise qvm-create-default-dvm may fail to include it in
saved-cows.tar, which will lead to DispVM being not really disposable.

Fixes QubesOS/qubes-issues#2200

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