virsh: Fix virsh prompt when connection changes to readonly mode.

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>