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/graphics-spice-auto-socket.x86_64-latest.args

@@ -26,9 +26,9 @@ 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"}' \
 -audiodev '{"id":"audio1","driver":"spice"}' \
 -spice unix,addr=/tmp/lib/domain--1-QEMUGuest1/spice.sock,seamless-migration=on \
--device cirrus-vga,id=video0,bus=pci.0,addr=0x2 \
+-device '{"driver":"cirrus-vga","id":"video0","bus":"pci.0","addr":"0x2"}' \
 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny \
 -msg timestamp=on