Update malware-analysis SDO's av_result property

[chisholm]

From oasis-tcs/cti-stix2#213
Fixes half of #339 .

Replace the av_result property with result and result_name properties.

[codecov-io]

Codecov Report

Merging #357 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #357      +/-   ##
=========================================
- Coverage   98.15%   98.1%   -0.05%     
=========================================
  Files         124     124              
  Lines       14176   13859     -317     
=========================================
- Hits        13914   13597     -317     
  Misses        262     262

Impacted Files	Coverage Δ
stix2/test/v21/test_malware_analysis.py	100% <ø> (ø)	⬆️
stix2/v21/sdo.py	94.53% <100%> (ø)	⬆️
stix2/datastore/filesystem.py	92.88% <0%> (-0.12%)	⬇️
stix2/patterns.py	98% <0%> (-0.02%)	⬇️
stix2/test/v21/test_datastore_filesystem.py	99.17% <0%> (-0.02%)	⬇️
stix2/test/v20/test_datastore_filesystem.py	99.19% <0%> (-0.02%)	⬇️
stix2/test/v21/test_relationship.py	100% <0%> (ø)	⬆️
stix2/test/v20/test_campaign.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_core.py	100% <0%> (ø)	⬆️
stix2/test/v21/test_attack_pattern.py	100% <0%> (ø)	⬆️
... and 59 more

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[clenk]

LGTM!