irule to select pool based on fqdn

You can use a if statement:

when HTTP_REQUEST {
  if { [HTTP::host] eq "web1.xyz.com" } {
    pool poolA
  } elseif { [HTTP::host] eq "web2.xyz.com" } {
    pool poolB
  }
}

or with a switch statement:

 when HTTP_REQUEST {
  switch [HTTP::host] {
    "web1.xyz.com" { 
      pool poolA
    }
    "web2.xyz.com" {
      pool poolB
    }
  }
}

I've been told that switch statements are let CPU intensive but it's really probably a toss in this situation. Use which ever method is easier for you to follow

-Joe