pci: Sanity test minimum downstream LNKSTA

pci: Sanity test minimum downstream LNKSTA

The entire link status register for SR-IOV VFs is defined as RsvdZ,
reads simply return zero.  Usually this is nothing more than lspci
reporting inconsequentially broken values:

    LnkSta: Speed unknown, Width x0, ...

However, now that we're using the downstream endpoint link status to
fill in the value at the parent downstream port, invalid values become
a problem.  In particular, the PCIe hotplug driver in Linux looks for
a valid negotiated link width and will fail to enumerate hot-added
downstream endpoints without non-zero value here, ex:

    pciehp 0000:00:02.0:pcie004: Slot(0): Attention button pressed
    pciehp 0000:00:02.0:pcie004: Slot(0) Powering on due to button press
    pciehp 0000:00:02.0:pcie004: Slot(0): Card present
    pciehp 0000:00:02.0:pcie004: Slot(0): Link Up
    pciehp 0000:00:02.0:pcie004: link training error: status 0x2000
    pciehp 0000:00:02.0:pcie004: Failed to check link status

Resolve by using minimum width and speed values for the downstream
port link status when the endpoint fails to provide valid values.
Long term, we may want to implement emulation in the vfio-pci host
driver to suppliment this field with the PF value as the SR-IOV spec
seems to allow, but the solution here is compatible should that be
implemented later.

Fixes: 727b48661f75 ("pci: Sync PCIe downstream port LNKSTA on read")
Reported-by: Jens Freimann <jfreimann@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <155060310248.19547.14979269067689441201.stgit@gimli.home>
Tested-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>