Update: use json to save opae.io driver state

[tswhison]

Description

opae.io init unbinds the target device from its driver and writes
a breadcrumb out to file so that the driver can be restored during
opae.io release. Previously, we were using the pickle module to do
this.

This github issue #3024 was filed, noting that pickle is prone to
code execution injection issues. The proposal was to use json
instead.

Collateral (docs, reports, design examples, case IDs):

#3024

• Document Update Required? (Specify FIM/AFU/Scripts)

Tests added:

Tests run:

Manual verification using opae.io init followed by opae.io release.

[coveralls]

Pull Request Test Coverage Report for Build 6502084603

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 68.399%

---

Files with Coverage Reduction	New Missed Lines	%
binaries/fpgad/monitor_thread.c	1	85.57%
libraries/plugins/xfpga/mmio.c	2	84.72%

Totals
Change from base Build 6501241052:	0.05%
Covered Lines:	15768
Relevant Lines:	23053

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💛 - Coveralls

[michael-adler]

Neat that replacing pickle with json is basically a switch of I/O packages, still using their dump/load methods.