virsh: Fix virsh prompt when connection changes to readonly mode.
    
    This commit fixes the virsh prompt when reconnection to the same URI is
    called: `virsh # connect --readonly` (Reconnect). The problem is
    happening because the code is considering URI (name) as a mandatory
    parameter to change the prompt. This commit remove the assignment into
    `priv->readonly` from `if (name)` conditional.
    
    Before:
        virsh # uri
        qemu:///system
    
        virsh # connect --readonly
    
        virsh #
    
    After:
        virsh # uri
        qemu:///system
    
        virsh # connect --readonly
    
        virsh >
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1507737
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>