PCI: Expose PCIe Resizable BAR support via sysfs

Add a simple sysfs interface to Resizable BAR support, largely for the
purposes of assigning such devices to a VM through VFIO.  Resizable BARs
present a difficult feature to expose to a VM through emulation, as
resizing a BAR is done on the host.  It can fail, and often does, but we
have no means via emulation of a PCIe REBAR capability to handle the error
cases.

A vfio-pci specific ioctl interface is also cumbersome as there are often
multiple devices within the same bridge aperture and handling them is a
challenge.  In the interface proposed here, expanding a BAR potentially
requires such devices to be soft-removed during the resize operation and
rescanned after, in order for all the necessary resources to be released.
A pci-sysfs interface is also more universal than a vfio specific
interface.

Please see the ABI documentation update for usage.

Link: https://lore.kernel.org/r/166336088796.3597940.14973499936692558556.stgit@omen
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Cc: Krzysztof Wilczyński <kw@linux.com>

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