pci_handler: Add pci_handler

Taken most of the logic from phoshpor-ipmi-flash:
https://github.com/openbmc/phosphor-ipmi-flash/blob/master/tools/io.cpp

Tested: Unit Test + Tested this on a real mach

pci_handler: Add pci_handler

Taken most of the logic from phoshpor-ipmi-flash:
https://github.com/openbmc/phosphor-ipmi-flash/blob/master/tools/io.cpp

Tested: Unit Test + Tested this on a real machine

Signed-off-by: Brandon Kim <brandonkim@google.com>
Change-Id: I546d1dff8764e31c9f2fa6c7465ba23a24f6fdea

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