qemu: capabilities: Enable detection of QEMU_CAPS_OBJECT_QAPIFIED

Base the detection on the presence of the 'secret' qom-type entry, which
isn't conditionally compiled in qemu.

All caps-based test now switch to using JSON for -object.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/qemuxml2argvdata/net-vdpa.x86_64-latest.args

@@ -9,12 +9,12 @@ XDG_CONFIG_HOME=/tmp/lib/domain--1-QEMUGuest1/.config \
 /usr/bin/qemu-system-i386 \
 -name guest=QEMUGuest1,debug-threads=on \
 -S \
--object secret,id=masterKey0,format=raw,\
-file=/tmp/lib/domain--1-QEMUGuest1/master-key.aes \
+-object '{"qom-type":"secret","id":"masterKey0","format":"raw",\
+"file":"/tmp/lib/domain--1-QEMUGuest1/master-key.aes"}' \
 -machine pc,accel=tcg,usb=off,dump-guest-core=off,memory-backend=pc.ram \
 -cpu qemu64 \
 -m 214 \
--object memory-backend-ram,id=pc.ram,size=224395264 \
+-object '{"qom-type":"memory-backend-ram","id":"pc.ram","size":224395264}' \
 -overcommit mem-lock=off \
 -smp 1,sockets=1,cores=1,threads=1 \
 -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \