restapi: version values for hosts are wrong

When the host is being installed or is being rebooted
the version value is unknow and is initialized to -1.
Displaying this in the /api/hosts xml results in schema
validation errors.

This patch supresses the version information if it is
unknown.

Change-Id: Ib6cf8f5480b29e07b60f36465b8fa0db74ee5246
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=891301
Signed-off-by: Ravi Nori <rnori@redhat.com>