qemu: driver: Document qemuOpenFile
qemu: driver: Document qemuOpenFile

The function is nontrivial to follow and has non-standard return values.
Recent usage was buggy.

zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jfehlig/libvirt,eskultety/libvirt,libvirt/libvirt,datto/libvirt,eskultety/libvirt,jfehlig/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,eskultety/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,crobinso/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,datto/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt,libvirt/libvirt,datto/libvirt,zippy2/libvirt,fabianfreyer/libvirt,datto/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,eskultety/libvirt,eskultety/libvirt,datto/libvirt,jardasgit/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt