qemublocktest: Add JSON->JSON test cases for block device backends

qemublocktest: Add JSON->JSON test cases for block device backends

Add testing of the interpretation of the JSON pseudo-protocol backing
store into JSON structs for blockdev. This will be used to test JSON
pseudo-URIs used by libguestfs while actually also validating the output
against the QMP schema. Since libguestfs uses obsolete/undocumented
values the outputs will differ and a benefit is that modern output is
used now.

The example test case covers the fields and values used by libguestfs
when using the https driver.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>