change argument to vm_config_file so it isn't the same as variable in powershell script.

plugins/providers/hyperv/action/import.rb

@@ -119,7 +119,7 @@ module VagrantPlugins
           # We have to normalize the paths to be Windows paths since
           # we're executing PowerShell.
           options = {
-            vm_config:  config_path.to_s.gsub("/", "\\"),
+            vm_config_file:  config_path.to_s.gsub("/", "\\"),
             vm_config_type:  config_type,
             image_path:      image_path.to_s.gsub("/", "\\")
           }

plugins/providers/hyperv/scripts/import_vm_vmcx.ps1

@@ -1,6 +1,6 @@
 Param(
     [Parameter(Mandatory=$true)]
-    [string]$vm_config,
+    [string]$vm_config_file,
     [Parameter(Mandatory=$true)]
     [string]$image_path,
 

plugins/providers/hyperv/scripts/import_vm_xml.ps1

@@ -1,6 +1,6 @@
 Param(
     [Parameter(Mandatory=$true)]
-    [string]$vm_config,
+    [string]$vm_config_file,
     [Parameter(Mandatory=$true)]
     [string]$image_path,