util: use string libvirt to prefix error message instead of libvir

util: use string libvirt to prefix error message instead of libvir

BZ:https://bugzilla.redhat.com/show_bug.cgi?id=912021
Without error handler set, virDefaultErrorFunc will be called, the
error message is prefixed with "libvir:". It become a little better
by using prefix "libvirt:" when working with upper application.

For example:
1, stop libvirtd daemon
2, run virt-top.
libvir: XML-RPC error : Failed to connect \
        socket to '/var/run/libvirt/libvirt-sock-ro': \
        No such file or directory
libvirt: VIR_ERR_SYSTEM_ERROR: VIR_FROM_RPC: \
        Failed to connect socket to '/var/run/libvirt/libvirt-sock-ro': \
        No such file or directory