Avoid apt seccomp issues due to libnss-libvirt

libnss-libvirt once enabled and with an existing guest causes apt to call
getdents (via apt -> nss -> libnss-libvirt).
which in the now seccomp enabled apt breaks almost everything related to apt.
Avoid this being an issue by dropping an apt conf that allows this as
suggested by Julian Andreas Klode (LP: #1732030).

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>