qemuBuildVsockCommandLine: Generate via JSON

qemuBuildVsockCommandLine: Generate via JSON

Build the properties of 'vhost-vsock' device via JSON. In comparison to
previous similar refactors this also modifies the hotplug code to attach
the vhost fd handle explicitly rather than using
'qemuMonitorAddDeviceWithFd'.

The properties of vhost-vsock have the following types according to
QEMU:

  guest-cid=<uint64>     -  (default: 0)
  vhostfd=<str>

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