Update document about qemu builder for [GH-1407].

https://github.com/mitchellh/packer/pull/1407

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

@@ -155,6 +155,12 @@ each category, the available options are alphabetized and described.
   must point to the same file (same checksum). By default this is empty
   and `iso_url` is used. Only one of `iso_url` or `iso_urls` can be specified.
 
+* `machine_type` (string) - Select the emulated machine by name.
+  Use "qemu-system-x86_64 -machine help"(Ubuntu),
+  "/usr/libexec/qemu-kvm -machine help"(RHEL7) to list available machines.
+  The Qemu default uses "pc-1.0" by default. If you use packer on RHEL7,
+  specify this value to "pc".
+
 * `net_device` (string) - The driver to use for the network interface. Allowed
   values "ne2k_pci," "i82551," "i82557b," "i82559er," "rtl8139," "e1000,"
   "pcnet" or "virtio." The Qemu builder uses "virtio" by default.