Fixes #3 via minor documentation fix and setting default properly (in the...

Fixes #3 via minor documentation fix and setting default properly (in the net_device template value, virtio is incorrect -- must be virtio-net).

builder/qemu/builder.go

@@ -168,7 +168,7 @@ func (b *Builder) Prepare(raws ...interface{}) error {
 	}
 
 	if b.config.NetDevice == "" {
-		b.config.NetDevice = "virtio"
+		b.config.NetDevice = "virtio-net"
 	}
 
 	if b.config.DiskInterface == "" {

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

@@ -47,7 +47,7 @@ paths to files, URLS for ISOs and checksums.
       "ssh_port": 22,
       "ssh_wait_timeout": "90m",
       "vm_name": "tdhtest",
-      "net_device": "virtio",
+      "net_device": "virtio-net",
       "disk_interface": "virtio",
       "boot_command":
       [
@@ -227,7 +227,16 @@ Optional:
   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
   switch/value pairs. Any value specified as an empty string is ignored.
-  All values after the switch are concatenated with no separater. For instance:
+  All values after the switch are concatenated with no separater.
+
+  WARNING: The qemu command line allows extreme flexibility, so beware of
+  conflicting arguments causing failures of your run. For instance, using
+   --no-acpi could break the ability to send power signal type commands (e.g.,
+  shutdown -P now) to the virtual machine, thus preventing proper shutdown. To
+  see the defaults, look in the packer.log file and search for the
+  qemu-system-x86 command. The arguments are all printed for review.
+
+  The following shows a sample usage:
 
 <pre class="prettyprint">
   . . .
@@ -251,11 +260,6 @@ Optional:
     qemu-system-x86 -m 1024m --no-acpi -netdev user,id=mynet0,hostfwd=hostip:hostport-guestip:guestport -device virtio-net,netdev=mynet0"
 </pre>
 
-  Note that the qemu command line allows extreme flexibility, so beware of
-  conflicting arguments causing failures of your run. To see the defaults,
-  look in the packer.log file and search for the qemu-system-x86 command. The
-  arguments are all printed for review.
-
 * `output_directory` (string) - This is the path to the directory where the
   resulting virtual machine will be created. This may be relative or absolute.
   If relative, the path is relative to the working directory when `packer`