libxl_driver: Resolve Coverity errors
libxl_driver: Resolve Coverity errors

1. The virObjectLock() call was unconditional, but Unlock was conditional
   on vm being valid.  Removed the check

2. A call to virDomainEventNewFromObj() isn't guaranteed to return an
   event - that check needs to be made prior to libxlDomainEventQueue()
   of the event. Did not add libxlDriverLock/Unlock around the call since
   some callers already have lock taken

3. Need to initialize fd = -1 in libxlDoDomainSave() since we can jump
   to cleanup before it's set.

4. Missing break;'s in libxlDomainModifyDeviceFlags() for case
   LIBXL_DEVICE_UPDATE.  The default: case would report an error

zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,crobinso/libvirt,datto/libvirt,agx/libvirt,jardasgit/libvirt,nertpinx/libvirt,taget/libvirt,agx/libvirt,elmarco/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,nertpinx/libvirt,cbosdo/libvirt,jfehlig/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,elmarco/libvirt,shugaoye/libvirt,agx/libvirt,jfehlig/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,olafhering/libvirt,elmarco/libvirt,rlaager/libvirt,jardasgit/libvirt,olafhering/libvirt,agx/libvirt,elmarco/libvirt,rlaager/libvirt,jardasgit/libvirt,eskultety/libvirt,VenkatDatta/libvirt,agx/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jfehlig/libvirt,cbosdo/libvirt,andreabolognani/libvirt,crobinso/libvirt,cbosdo/libvirt,andreabolognani/libvirt,elmarco/libvirt,rlaager/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,taget/libvirt,olafhering/libvirt,VenkatDatta/libvirt,taget/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,libvirt/libvirt,datto/libvirt,nertpinx/libvirt,libvirt/libvirt,cbosdo/libvirt,taget/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,taget/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,eskultety/libvirt,crobinso/libvirt,zhlcindy/libvirt-1.1.4-maintain,datto/libvirt,eskultety/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,datto/libvirt,rlaager/libvirt,rlaager/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,eskultety/libvirt,libvirt/libvirt