Replace subdomain and remove path

e.g.

 config

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm virtual bar
ltm virtual bar {
    destination 172.28.24.10:80
    ip-protocol tcp
    mask 255.255.255.255
    pool foo
    profiles {
        http { }
        tcp { }
    }
    rules {
        myrule
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    vs-index 2
}
root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm pool foo
ltm pool foo {
    members {
        200.200.200.101:80 {
            address 200.200.200.101
        }
    }
}
root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm rule myrule
ltm rule myrule {
    when HTTP_REQUEST {
  if { [HTTP::path] starts_with "/customfiles/" } {
    HTTP::uri [string map {/customfiles/ /} [HTTP::uri]]
    HTTP::header replace Host "customfiles.mysite.org"
  }
}
}

 trace

[root@ve11a:Active:In Sync] config  ssldump -Aed -nni 0.0 port 80
New TCP connection 1: 172.28.24.1(36831) <-> 172.28.24.10(80)
1391521795.0090 (0.0011)  C>S
---------------------------------------------------------------
GET /customfiles/mydoc.htm HTTP/1.1
User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5
Accept: */*
Host: www.mysite.org

---------------------------------------------------------------

New TCP connection 2: 200.200.200.14(36831) <-> 200.200.200.101(80)
1391521795.0275 (0.0173)  C>S
---------------------------------------------------------------
GET /mydoc.htm HTTP/1.1
User-Agent: curl/7.15.5 (i686-redhat-linux-gnu) libcurl/7.15.5 OpenSSL/0.9.8b zlib/1.2.3 libidn/0.6.5
Accept: */*
Host: customfiles.mysite.org

---------------------------------------------------------------