backups: do not hardcode limit values in the code (#771)
backups: do not hardcode limit values in the code (#771)

Also introduce limit for stderr read from VM (anti DoS protection).

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