Enhance the VIF IP configuration feature

[dinhngtu]

1. Add the ability to configure DNS settings for VIF configuration.

The static IP configuration mode now accepts a dns=a.a.a.a,b.b.b.b setting that sets the DNS server on the target VIF.

The Xenstore schema is as following:

• xenserver/device/vif/<id>/static-ip-setting/
  • dns/
    • 0 = "198.51.100.53"
    • 1 = "198.51.100.54"
  • dns6/
    • 0 = "2001:db8::53"
    • 1 = "2001:db8::54"

If the dns or dns6 key is missing, then it should be interpreted as "don't change the corresponding DNS settings". This retains compatibility with guest agents that don't support DNS setting.

If the dns or dns6 key is present but contains no children, then it should be interpreted as "empty the list of DNS servers".

The latter (empty the list of DNS servers) can be presented in xenstore, but cannot be represented in XAPI DB or xe, because XAPI DB does not have a way to represent null values.

2. Wait for VIF IP configuration to complete and report the error

During VIF.configure_ipv4/ipv6, XAPI simply sets the xenstore entries and finish immediately. This doesn't tell us if the configuration succeeded or not.

Instead, use the following protocol:

• Guest agent sets xenserver/device/vif/<id>/static-ip-setting/error-code and error-msg with the results. error-msg is ignored if error-code=0.
• Guest agent removes the corresponding enabled or enabled6 to acknowledge the configuration to XAPI
• XAPI reads error-code and error-msg to report back to the caller