qemuBuildShmemCommandLine: Generate via JSON

qemuBuildShmemCommandLine: Generate via JSON

Note that the legacy 'ivshmem' device was already removed upstream, but
it's converted so that the code is identical.

For the two modern devices QEMU considers the properties being of
following types:

'ivshmem-doorbell'
  chardev=<str>          - ID of a chardev to use as a backend
  ioeventfd=<bool>       - on/off (default: true)
  master=<OnOffAuto>     - on/off/auto (default: "off")
  vectors=<uint32>       -  (default: 1)

'ivshmem-plain'
  master=<OnOffAuto>     - on/off/auto (default: "off")
  memdev=<link<memory-backend>>

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