[PPC] Remove extra \0 char inserted on vty by QEMU

Since version 2.11.0, QEMU became bug-compatible with
PowerVM's vty implementation, by inserting a \0 after
every \r going to the guest. Guests are expected to
workaround this issue by removing every \0 immediately
following a \r.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22171