qemu: capabilities: Always assume QEMU_CAPS_BOOTINDEX

The field was added in qemu v0.13.0-rc0-731-g1ca4d09ae0 so all supported
qemu versions now use it.

There's a LOT of test fallout as we did not use capabilities close
enough to upstream for many of our tests.

Several tests had a 'bootindex' variant. Since they'd become redundant
they are also removed here.

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

tests/qemuxml2argvdata/virtio-rng-random.args

@@ -20,7 +20,6 @@ server,nowait \
 -rtc base=utc \
 -no-shutdown \
 -no-acpi \
--boot c \
 -usb \
 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3 \
 -object rng-random,id=objrng0,filename=/dev/urandom \