restapi : version values for hosts are wrong for libvirt

When a new host is added to engine and the status is either
installing for rebooting the libvirt version is unknown and
is reported as -1.

This patch supresses the libvirt version from /api/hosts if the
version is unknown.

Change-Id: I0c0525b6bb8c90d2ce4e10ac4f5c26f1a1b1e6b0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=904935
Signed-off-by: Ravi Nori <rnori@redhat.com>