qemu: capabilities: Re-enable JSON syntax for -device

Now that qemu fixed device unplug when JSON syntax is used with -device
we can re-enable the feature.

Since the old capability string representation is condemned by
suggesting filtering it as a workaround we must introduce a new string.
To achieve this the original capability position is renamed to
X_QEMU_CAPS_DEVICE_JSON_BROKEN_HOTPLUG and a new position with the
original name QEMU_CAPS_DEVICE_JSON is introduced to prevent us having
to change the rest of the code.

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

tests/qemuxml2argvdata/virtio-options-net-iommu.x86_64-latest.args

@@ -27,12 +27,12 @@ XDG_CONFIG_HOME=/tmp/lib/domain--1-QEMUGuest1/.config \
 -no-shutdown \
 -no-acpi \
 -boot strict=on \
--device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 \
+-device '{"driver":"piix3-usb-uhci","id":"usb","bus":"pci.0","addr":"0x1.0x2"}' \
 -netdev user,id=hostnet0 \
--device virtio-net-pci,iommu_platform=on,netdev=hostnet0,id=net0,mac=52:54:56:58:5a:5c,bus=pci.0,addr=0x2 \
+-device '{"driver":"virtio-net-pci","iommu_platform":true,"netdev":"hostnet0","id":"net0","mac":"52:54:56:58:5a:5c","bus":"pci.0","addr":"0x2"}' \
 -netdev user,id=hostnet1 \
--device virtio-net-pci,iommu_platform=off,netdev=hostnet1,id=net1,mac=62:64:66:68:6a:6c,bus=pci.0,addr=0x3 \
+-device '{"driver":"virtio-net-pci","iommu_platform":false,"netdev":"hostnet1","id":"net1","mac":"62:64:66:68:6a:6c","bus":"pci.0","addr":"0x3"}' \
 -audiodev '{"id":"audio1","driver":"none"}' \
--device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 \
+-device '{"driver":"virtio-balloon-pci","id":"balloon0","bus":"pci.0","addr":"0x4"}' \
 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
 -msg timestamp=on