arm/arm64: vfio: Add PCI Express Capability Structure

It turns out that some Linux drivers (like Realtek R8169) fall back to a
device-specific configuration method if the device is not PCI Express
capable:

[    1.433825] r8169 0000:00:00.0 enp0s0: No native access to PCI extended config space, falling back to CSI

Add the PCI Express Capability Structure and populate it for assigned
devices, as this is how the Linux PCI driver determines if a device is PCI
Express capable.

Because we don't emulate a PCI Express link, a root complex or any slot
related properties, the PCI Express capability is kept as small as possible
by ignoring those fields.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>