Fix a bug for the config type in the Set-AzureVMAccessExtension cmdlet
Rename a helper function name for getting the config for VMAccess extension
Fix New-AzureServiceADDomainExtensionConfig returns with "Parameter set cannot be resolved." #2276

```
PS> New-AzureServiceADDomainExtensionConfig -ThumbprintAlgorithm "sha1" -Restart -Credential $cred -DomainName "djtest.com" -CertificateThumbprint "1982B518A81462A5596B8FCAE46824076B95C647"

ProviderNameSpace     : Microsoft.Windows.Azure.Extensions
Type                  : ADDomain
Version               :
CertificateThumbprint : 1982B518A81462A5596B8FCAE46824076B95C647
ThumbprintAlgorithm   : sha1
PublicConfiguration   : <?xml version="1.0" encoding="utf-16"?>
                        <PublicConfig xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                          <Name>djtest.com</Name>
                          <User>ttt</User>
                          <Options>1</Options>
                          <Restart>true</Restart>
                        </PublicConfig>
PrivateConfiguration  : <?xml version="1.0" encoding="utf-16"?>
                        <PrivateConfig xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
                          <Password>ttt</Password>
                          <UnjoinDomainPassword />
                        </PrivateConfig>
Roles                 : {Default}
X509Certificate       :
```