Add support for dedicated XML agent support (with TLS)

[mirceaulinic]

Original ticket from napalm-iosxr: napalm-automation/napalm-iosxr#43

It is supposedly faster than TTY agent.
http://www.cisco.com/c/en/us/td/docs/ios_xr_sw/iosxr_r4-1/xml/programming/guide/xl41apidoc/xl41noti.html#wp1068023