[pci] Backup and restore standard config space across PCIe FLR

[pci] Backup and restore standard config space across PCIe FLR

The behaviour of PCI devices across a function-level reset seems to be
inconsistent in practice: some devices will preserve PCI BARs, some
will not.

Fix the behaviour of FLR on devices that do not preserve PCI BARs by
backing up and restoring PCI configuration space across the reset.
Preserve only the standard portion of the configuration space, since
there may be registers with unexpected side effects in the remaining
non-standardised space.

Signed-off-by: Michael Brown <mcb30@ipxe.org>