PCI: Allow userspace to query and set device reset mechanism

Add "reset_method" sysfs attribute to enable user to query and set
preferred device reset methods and their ordering.

[bhelgaas: on invalid sysfs input, return error and preserve previous
config, as in earlier patch versions]
Co-developed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20210817180500.1253-6-ameynarkhede03@gmail.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Amey Narkhede <ameynarkhede03@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>