pci passthrough fails while trying to decode extra_info

pci device with empty extra_info should use null dict instead of
None. keep the init of extra_info to make db could access it.

Closes-Bug: #1223559

Change-Id: I2e57eb53dfd4f5168d52dc94c2057fc575458aee
Signed-off-by: Yongli He <yongli.he@intel.com>
Merge "pci passthrough fails while trying to decode extra_info"