backup: don't crash when no 'qubes' group is present
backup: don't crash when no 'qubes' group is present

This is specifically the case on Travis-CI. But since dom0 backup is
restored into separate directory now, instead of directly overriding
user home, this check is much less relevant now.

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client