properly encode YAML via YAML::XS

properly encode YAML via YAML::XS

otherwise we get strange errors when formatting data that was originally
JSON, and can thus contain JSON::true/JSON::false.

one example is the QMP query-blockstats command, which gets called (and
the resulting values returned) by /nodes/NODE/qemu/VMID/status/current

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>