qemu: command: Introduce JSON variant of qemuBuildVirtioDevStr

qemu: command: Introduce JSON variant of qemuBuildVirtioDevStr

Add a JSON variant of the generator of properties for virtio devices.
For convenience both the old and new are for now marked as unused, which
will be removed once the conversion is complete.

The formatted properties have following types according to QEMU.
'virtio-blk-pci' was used as an example:

  disable-legacy=<OnOffAuto> - on/off/auto (default: "auto")
  disable-modern=<bool>  -  (default: false)
  iommu_platform=<bool>  - on/off (default: false)
  ats=<bool>             - on/off (default: false)
  packed=<bool>          - on/off (default: false)

Note that <OnOffAuto> is an enum type without alternates in QMP so it
must be represented as a string in JSON.

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