qemu: Translate boot config into bootindex if possible

Prefer bootindex=N option for -device over the old way -boot ORDER
possibly accompanied with boot=on option for -drive. This gives us full
control over which device will actually be used for booting guest OS.
Moreover, if qemu doesn't support boot=on, this is the only way to boot
of certain disks in some configurations (such as virtio disks when used
together IDE disks) without transforming domain XML to use per device
boot elements.

tests/qemuxml2argvdata/qemuxml2argv-disk-drive-no-boot.args

@@ -0,0 +1,18 @@
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test \
+/usr/bin/qemu \
+-S \
+-M pc \
+-m 214 \
+-smp 1 \
+-nographic \
+-nodefaults \
+-monitor unix:/tmp/test-monitor,server,nowait \
+-no-acpi \
+-drive file=/dev/HostVG/QEMUGuest1,if=none,id=drive-ide0-0-0 \
+-device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=2 \
+-drive file=/dev/HostVG/QEMUGuest2,if=none,media=cdrom,id=drive-ide0-1-0 \
+-device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0,bootindex=1 \
+-drive file=/dev/fd0,if=none,id=drive-fdc0-0-0 \
+-global isa-fdc.driveA=drive-fdc0-0-0 \
+-global isa-fdc.bootindexA=3 \
+-usb