emulators/qemu-powernv: fix build with lld 10

Apply db@'s variant from r528777 to fix lld 10 build issues with qemu ports.
While here fix a port QA issue by adding libxml2 to USE_GNOME.

Approved by: timur (mentor)
Sponsored by: BBOX.io
Differential Revision: https://reviews.freebsd.org/D24390