virsh: Implement "domrename" command

virsh: Implement "domrename" command

This patch implements new virsh command, domrename.

Using domrename, it will be possible to rename domain from the virsh shell by
calling virRenameDomain API.

It takes two arguments, current domain name and new domain name.

Example:

virsh # list --all
 Id    Name                           State
 ----------------------------------------------------
  -     bar                            shut off

virsh # domrename bar foo
Domain successfully renamed

virsh # list --all
 Id    Name                           State
 ----------------------------------------------------
  -     foo                            shut off

virsh #

Signed-off-by: Tomas Meszaros <exo@tty.sk>