xen: convert "-machine igd-passthru" to an accelerator property

The first machine property to fall is Xen's Intel integrated graphics
passthrough.  The "-machine igd-passthru" option does not set anymore
a property on the machine object, but desugars to a GlobalProperty on
accelerator objects.

The setter is very simple, since the value ends up in a
global variable, so this patch also provides an example before the more
complicated cases that follow it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/qemu-options.hx b/qemu-options.hx
index ee1f676..1535390 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -37,7 +37,6 @@
     "                kvm_shadow_mem=size of KVM shadow MMU in bytes\n"
     "                dump-guest-core=on|off include guest memory in a core dump (default=on)\n"
     "                mem-merge=on|off controls memory merge support (default: on)\n"
-    "                igd-passthru=on|off controls IGD GFX passthrough support (default=off)\n"
     "                aes-key-wrap=on|off controls support for AES key wrapping (default=on)\n"
     "                dea-key-wrap=on|off controls support for DEA key wrapping (default=on)\n"
     "                suppress-vmdesc=on|off disables self-describing migration (default=off)\n"
@@ -71,8 +70,6 @@
 fails to initialize.
 @item kernel_irqchip=on|off
 Controls in-kernel irqchip support for the chosen accelerator when available.
-@item gfx_passthru=on|off
-Enables IGD GFX passthrough support for the chosen machine when available.
 @item vmport=on|off|auto
 Enables emulation of VMWare IO port, for vmmouse etc. auto says to select the
 value based on accel. For accel=xen the default is off otherwise the default
@@ -120,6 +117,7 @@
 DEF("accel", HAS_ARG, QEMU_OPTION_accel,
     "-accel [accel=]accelerator[,prop[=value][,...]]\n"
     "                select accelerator (kvm, xen, hax, hvf, whpx or tcg; use 'help' for a list)\n"
+    "                igd-passthru=on|off (enable Xen integrated Intel graphics passthrough, default=off)\n"
     "                tb-size=n (TCG translation block cache size)\n"
     "                thread=single|multi (enable multi-threaded TCG)\n", QEMU_ARCH_ALL)
 STEXI
@@ -130,6 +128,9 @@
 more than one accelerator specified, the next one is used if the previous one
 fails to initialize.
 @table @option
+@item igd-passthru=on|off
+When Xen is in use, this option controls whether Intel integrated graphics
+devices can be passed through to the guest (default=off)
 @item tb-size=@var{n}
 Controls the size (in MiB) of the TCG translation block cache.
 @item thread=single|multi