Description of problem:

we were trying to start pmatyas-host-01 and it failed, running later was ok. also something odd appeared in UI, but unfortunatelly I did not save it :/

2018-04-12 10:46:34,249+02 INFO  [org.ovirt.engine.core.vdsbroker.CreateVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] START, CreateVDSCommand( CreateVDSCommandParameters:{hostId='94083e18-550c-428c-8abc-0e4bc9839e4e
', vmId='eb91d238-1a2c-4588-8f22-73cac948006e', vm='VM [pmatyas-host-01]'}), log id: 657b2d31

...
2018-04-12 10:46:34,259+02 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.CreateBrokerVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] Failed in 'CreateBrokerVDS' method, for vds: 'slot-7b'; host: 'slot-7b.rhev.lab.e
ng.brq.redhat.com': null
2018-04-12 10:46:34,259+02 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.CreateBrokerVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] Command 'CreateBrokerVDSCommand(HostName = slot-7b, CreateVDSCommandParameters:{h
ostId='94083e18-550c-428c-8abc-0e4bc9839e4e', vmId='eb91d238-1a2c-4588-8f22-73cac948006e', vm='VM [pmatyas-host-01]'})' execution failed: null
2018-04-12 10:46:34,260+02 DEBUG [org.ovirt.engine.core.vdsbroker.vdsbroker.CreateBrokerVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] Exception: java.lang.NullPointerException
        at org.ovirt.engine.core.vdsbroker.builder.vminfo.LibvirtVmXmlBuilder.lambda$writeInterfaces$23(LibvirtVmXmlBuilder.java:1091) [vdsbroker.jar:]
        at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469) [rt.jar:1.8.0_161]
        at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355) [rt.jar:1.8.0_161]
        at java.util.TimSort.sort(TimSort.java:220) [rt.jar:1.8.0_161]
        at java.util.Arrays.sort(Arrays.java:1512) [rt.jar:1.8.0_161]
        at java.util.stream.SortedOps$SizedRefSortingSink.end(SortedOps.java:348) [rt.jar:1.8.0_161]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) [rt.jar:1.8.0_161]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) [rt.jar:1.8.0_161]
        at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151) [rt.jar:1.8.0_161]
        at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174) [rt.jar:1.8.0_161]
...
2018-04-12 10:46:34,286+02 ERROR [org.ovirt.engine.core.vdsbroker.CreateVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] Command 'CreateVDSCommand( CreateVDSCommandParameters:{hostId='94083e18-550c-428c-8abc-0e4bc9839e4e', vmId='eb91d238-1a2c-4588-8f22-73cac948006e', vm='VM [pmatyas-host-01]'})' execution failed: java.lang.NullPointerException
2018-04-12 10:46:34,286+02 DEBUG [org.ovirt.engine.core.vdsbroker.CreateVDSCommand] (default task-7) [21c0e7ba-790c-4e00-af3e-b73aae692c9b] Exception: java.lang.RuntimeException: java.lang.NullPointerException
        at org.ovirt.engine.core.vdsbroker.CreateVDSCommand.executeVmCommand(CreateVDSCommand.java:68) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.ManagingVmCommand.executeVDSCommand(ManagingVmCommand.java:17) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.VDSCommandBase.executeCommand(VDSCommandBase.java:73) [vdsbroker.jar:]
        at org.ovirt.engine.core.dal.VdcCommandBase.execute(VdcCommandBase.java:33) [dal.jar:]
        at org.ovirt.engine.core.vdsbroker.vdsbroker.DefaultVdsCommandExecutor.execute(DefaultVdsCommandExecutor.java:14) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.ResourceManager.runVdsCommand(ResourceManager.java:398) [vdsbroker.jar:]
        at org.ovirt.engine.core.vdsbroker.ResourceManager$Proxy$_$$_WeldSubclass.runVdsCommand$$super(Unknown Source) [vdsbroker.jar:]
        at sun.reflect.GeneratedMethodAccessor91.invoke(Unknown Source) [:1.8.0_161]
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.8.0_161]
        at java.lang.reflect.Method.invoke(Method.java:498) [rt.jar:1.8.0_161]

Version-Release number of selected component (if applicable):
ovirt-engine-4.2.2.6-0.1.el7.noarch

How reproducible:
1 in 1

Steps to Reproduce:
1. 
2.
3.

Actual results:
a vm could not be started, odd output in ui and jnpe in engine.log

Expected results:
at least clear message why it could not run

Additional info: