udev: only report a warning if udev_enumerate_scan_devices fails
udev: only report a warning if udev_enumerate_scan_devices fails

Even if an error is reported by `udev_enumerate_scan_devices`,
e.g. because a driver of a device has an bug, we can still enumerate
all other devices. Additionally the documentation of
udev_enumerate_scan_devices says that on success an integer >= 0 is
returned (see man udev_enumerate_scan_devices(3)).

Reviewed-by: Bjoern Walk <40545c009452f40d2ae0bbfbd987b332d274d328@linux.ibm.com>
Signed-off-by: Marc Hartmayer <7bafc8cd4e6394916d396e4f8cde69753be8d066@linux.ibm.com>
Reviewed-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

fabianfreyer/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,eskultety/libvirt,andreabolognani/libvirt,eskultety/libvirt,crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,zippy2/libvirt,nertpinx/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt,olafhering/libvirt,jardasgit/libvirt,libvirt/libvirt,eskultety/libvirt,jfehlig/libvirt