A Dynamips Ethernet port can anchor a kernel link whenever its compute's
uBridge has the tap module — the anchors are uBridge-created TAPs the
hypervisor merely opens, the same capability QEMU gates on, so no
Dynamips-specific capability is needed. Serial, ATM and POS ports are
excluded per port by the existing link-type check (the controller's port
matrix decides), keeping them on the relay whatever the capabilities.
Eligibility tests cover the mixed pairs (docker/qemu/iou endpoints on the
same compute), the strict tap-capability requirement, and the serial
exclusion.