Fix up the failure case of dom.id.
Fix up the failure case of dom.id.

Coverity pointed out that id, as returned by virDomainGetID,
is in fact an unsigned int.  That means we can't compare
against -1 to find out if it fails.  We really need to
cast -1 to unsigned int to make sure we are comparing against
the right thing.

Signed-off-by: Chris Lalancette <60b62644009db6b194cc0445b64e9b27bb26433a@redhat.com>

pronix/ruby-libvirt,pronix/ruby-libvirt,pronix/ruby-libvirt