The qemu-img info command shows the encrpyed field in different formats
according to the output format. When the default human format is used
the field can be 'yes' or None while when json format is used the same
field can be True or False.
This change ensures the corresponding attribute has the consistent
format regardless of the output format used.
Closes-Bug: #1942682
Change-Id: I949f07582a708114fdfba76f1a05aa3a3e5c4f40