net: source route: don't attempt to configure invalid values

If a DHCP server provides Vdsm with invalid ip/netmask/gateway supervdsm
logs a not-very-useful

    "ipaddr, mask or gateway not received"

Instead, this patch logs dhclient's faulty information before passing it
to the source route configurator, which hopefully would provide the
local admin more clues regarding the failure.

Change-Id: I0c4719629bfdc632bbc35171489670062c6c14cb
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34393
Tested-by: Ido Barkan <ibarkan@redhat.com>