vnc: add a more descriptive error message

Currently qemu outputs some low-level error in qemu-sockets.c
when failed to start vnc server.
eg. 'getaddrinfo(127.0.0.1,5902): Name or service not known'

Some libvirt users could not know what's happened with this
unclear error message. This patch added a more descriptive
error message.

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>