qemu: Add Taint for nbdkit restart failure

qemu: Add Taint for nbdkit restart failure

Since the restart handler will trigger at an arbitrary time (when the
nbdkit process crashes, for instance), it's difficult to provide
feedback to the user if the restart is unsuccessful. Rather than just
relying on a warning in the log, taint the domain so that there will be
a slightly more user-visible notification.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>