$vcen = "vCenterServer"
$xmlFile = "VMHost.xml"

Connect-VIServer -Server $vcen
 
#Get the File
[xml]$xml = Get-Content $xmlFile
 
#Populate the Fields
$xml.updatehostcustom.vmhost | foreach {
  Set-Annotation -Entity (get-vmhost $_.Name) `
      -CustomAttribute AssetTag -Value $_.AssetTag
  Set-Annotation -Entity (get-vmhost $_.Name) `
      -CustomAttribute ServiceTag -Value $_.ServiceTag
  Set-Annotation -Entity (get-vmhost $_.Name) `
      -CustomAttribute PurchaseDate -Value $_.PurchaseDate
  Set-Annotation -Entity (get-vmhost $_.Name) `
      -CustomAttribute WarrantyExpires -Value $_.WarrantyExpires
}

Disconnect-VIServer $vcen -Force -Confirm:$false