The NVMe CAM driver reports the PCIe Link Capability and Status for devices. For emulated bhyve NVMe devices, this looks like:
nda0: nvme version 1.3 x63 (max x63) lanes PCIe Gen15 (max Gen15) link
The driver outputs this because the emulated device doesn't include the PCIe Capability structure. The NVMe specification requires these registers, so the fix is to add this set of capability registers to the emulated device.
Note that PCI Express devices that are integrated into the Root Complex (i.e. Bus 0x0) do not have to support the Link Capability or Status registers. Windows will fail to start (i.e. Code 10) devices that appear to be part of the Root Complex but report being a PCI Express Endpoint. So also add a check to pci_emul_add_pciecap() to check if the device is integrated and change the device type.