Disassembler: Improve pathname support for emitted External() statements.

This change adds full pathname support for external names that have been
resolved internally by the inclusion of additional ACPI tables (via the
iASL -e option). Without this change, the disassembler can emit multiple
externals for the same object, or it become confused when the Scope()
operator is used on an external object. Reported by Michael Tsirkin
(mst@redhat.com).