Fix the error
```homestead-7: Creating and registering the VM...
An error occurred while executing a PowerShell script. This error
is shown below. Please read the error message and see if this is
a configuration error with your system. If it is not, then please
report a bug.
Script: import_vm_vmcx.ps1
Error:
At C:\Program Files
(x86)\HashiCorp\Vagrant\embedded\gems\gems\vagrant-1.9.4\plugins\providers\hyper v\scripts\import_vm_vmcx.ps1:18 char:37
+ [string]$differencing_disk=$null
+ ~
Missing ')' in function parameter list.
At C:\Program Files
(x86)\HashiCorp\Vagrant\embedded\gems\gems\vagrant-1.9.4\plugins\providers\hyper v\scripts\import_vm_vmcx.ps1:20 char:1
+ )
+ ~
Unexpected token ')' in expression or statement.
+ CategoryInfo : ParserError: (:) [], ParseException
+ FullyQualifiedErrorId : MissingEndParenthesisInFunctionParameterList
```
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