libvirt-sys-importer: import_domain() handle errors
libvirt-sys-importer: import_domain() handle errors

import_domain() should be handling it's own errors anyway but this is
also needed cause vala doesn't seem to copy/ref array of objects for
closure and hence we have a potential crash at our hand if the calling
function, import() handles the errors from import_domain()
asynchronously, where it accesses the domain config array.

vbenes/gnome-boxes,vbenes/gnome-boxes