Update xenserver-iso.html.markdown

Add xva_compressed to documentation

Author: AtaxyaNetwork

docs/builders/iso/xenserver-iso.html.markdown

@@ -102,7 +102,7 @@ each category, the available options are alphabetized and described.
   characters (\*, ?, and []) are allowed. Directory names are also allowed,
   which will add all the files found in the directory to the floppy.
 
-* `format` (string) - Either "xva", "vdi_raw" or "none", this specifies the
+* `format` (string) - Either "xva", "xva_compressed", "vdi_raw" or "none", this specifies the
   output format of the exported virtual machine. This defaults to "xva". Set to
   "vdi_raw" to export just the raw disk image. Set to "none" to export nothing;
   this is only useful with "keep_vm" set to "always" or "on_success".