virsh: Notify users about disconnects

virsh: Notify users about disconnects

After my "client rpc: Report proper error for keepalive disconnections"
patch, virsh would no long print a warning when it closes a connection
to a daemon after a keepalive timeout. Although the warning

    virsh # 2015-09-15 10:59:26.729+0000: 642080: info :
    libvirt version: 1.2.19
    2015-09-15 10:59:26.729+0000: 642080: warning :
    virKeepAliveTimerInternal:143 : No response from client
    0x7efdc0a46730 after 1 keepalive messages in 2 seconds

was pretty ugly, it was still useful. This patch brings the useful part
back while making it much nicer:

virsh # error: Disconnected from qemu:///system due to keepalive timeout

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>