virsh: Add domdisplay cmd for VNC, SPICE and RDP

virsh: Add domdisplay cmd for VNC, SPICE and RDP

v2:
- Refactored to use virBuffer
- Refactored to use virXPath wrappers
- Added support for tls-port and password for SPICE
- Added optional flag to disable SPICE password to the URI
- Added support for RDP
- Fixed code reviews

Add a new 'domdisplay' command that provides a URI for VNC, SPICE and
RDP connections. Presently the 'vncdisplay' command provides you with
the port info that QEMU is listening on but there is no counterpart for
SPICE and RDP. Additionally this provides you with the bind address as
specified in the XML, which the existing 'vncdisplay' lacks. For SPICE
connections it supports secure and unsecure channels and optionally
providing the password for the SPICE channel.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>