Add missing default values for 'disk_cache' and 'disk_discard' in qemu builder...

Add missing default values for 'disk_cache' and 'disk_discard' in qemu builder docs, sort qemu_binary before qemuargs so it matches ordering from 'packer fix'

website/source/docs/builders/qemu.html.markdown

@@ -117,10 +117,10 @@ can be configured for this builder.
 
 * `disk_cache` (string) - The cache mode to use for disk. Allowed values
   values include any of "writethrough", "writeback", "none", "unsafe" or
-  "directsync".
+  "directsync".  By default, this is set to "writeback".
 
 * `disk_discard` (string) - The discard mode to use for disk. Allowed values
-  include any of "unmap" or "ignore".
+  include any of "unmap" or "ignore".  By default, this is set to "ignore".
 
 * `disk_image` (boolean) - Packer defaults to building from an ISO file,
   this parameter controls whether the ISO URL supplied is actually a bootable
@@ -148,7 +148,7 @@ can be configured for this builder.
 * `format` (string) - Either "qcow2" or "raw", this specifies the output
   format of the virtual machine image. This defaults to "qcow2".
 
-* `headless` (boolean) - Packer defaults to building virtual machines by
+* `headless` (boolean) - Packer defaults to building QEMU virtual machines by
   launching a GUI that shows the console of the machine being built.
   When this value is set to true, the machine will start without a console.
 
@@ -188,6 +188,11 @@ can be configured for this builder.
   By default this is "output-BUILDNAME" where "BUILDNAME" is the name
   of the build.
 
+* `qemu_binary` (string) - The name of the Qemu binary to look for.  This
+  defaults to "qemu-system-x86_64", but may need to be changed for some
+  platforms.  For example "qemu-kvm", or "qemu-system-i386" may be a better
+  choice for some systems.
+
 * `qemuargs` (array of array of strings) - Allows complete control over
   the qemu command line (though not, at this time, qemu-img). Each array
   of strings makes up a command line switch that overrides matching default
@@ -225,11 +230,6 @@ qemu-system-x86 command. The arguments are all printed for review.
 	qemu-system-x86 -m 1024m --no-acpi -netdev user,id=mynet0,hostfwd=hostip:hostport-guestip:guestport -device virtio-net,netdev=mynet0"
 </pre>
 
-* `qemu_binary` (string) - The name of the Qemu binary to look for.  This
-  defaults to "qemu-system-x86_64", but may need to be changed for some
-  platforms.  For example "qemu-kvm", or "qemu-system-i386" may be a better
-  choice for some systems.
-
 * `shutdown_command` (string) - The command to use to gracefully shut down
   the machine once all the provisioning is done. By default this is an empty
   string, which tells Packer to just forcefully shut down the machine.