Add domain name check and UUID check to 'xm new' command.

Add domain name check and UUID check to 'xm new' command.

Add a domain name check and a UUID check to xm new command. The check
logic is as follows:

 - If the UUID is not specified
       - If a VM with same name exists
           => Update the config for that existing VM
       - Else no vm with same name exists
           => Define a brand new VM with auto-generated UUID
 - Else UUID is specified
       - If a VM with same UUID exists
             - If name is different
                   => Error
             - Else if name is same
                   => Update the config for that existing VM
       - Else no VM with same UUID exists
             - If name is different
                   => Define a branch new VM with that name
             - Else if name is same
                   => Error

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>