Man page: Add -global description

There's only TODO information in qemu man page for -global option. This is a basic description of this option with simple example.
Signed-off-by: NMiroslav Rezanina <mrezanin@redhat.com>

v4:
 - break long line

v3:
 - add use case description
 - use prop instead of property

v2:
 - Use better value in example
Patch:
--
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-options.hx

@@ -288,13 +288,21 @@ TODO
 ETEXI
 
 DEF("global", HAS_ARG, QEMU_OPTION_global,
-    "-global driver.property=value\n"
+    "-global driver.prop=value\n"
     "                set a global default for a driver property\n",
     QEMU_ARCH_ALL)
 STEXI
-@item -global
+@item -global @var{driver}.@var{prop}=@var{value}
 @findex -global
-TODO
+Set default value of @var{driver}'s property @var{prop} to @var{value}, e.g.:
+
+@example
+qemu -global ide-drive.physical_block_size=4096 -drive file=file,if=ide,index=0,media=disk
+@end example
+
+In particular, you can use this to set driver properties for devices which are 
+created automatically by the machine model. To create a device which is not 
+created automatically and set properties on it, use -@option{device}.
 ETEXI
 
 DEF("mtdblock", HAS_ARG, QEMU_OPTION_mtdblock,