backup: check if there is enough disk space available to restore
backup: check if there is enough disk space available to restore

Avoid failing restore after potentially lengthy processing - check
available space first.

This will be even more important after adding waiting for free space -
then if there won't be enough free space, the process will deadlock.

QubesOS/qubes-issues#4791

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