vm/file-editor: connect /dev/null as stdin (#657)
vm/file-editor: connect /dev/null as stdin (#657)

If stdin is closed (not connected to /dev/null) first opened file will get fd=0
and will be treated as stdin.

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