UML: console should handle spurious IRQS

The previous DEBUG_SHIRQ patch missed one case.  The console doesn't
set its host descriptors non-blocking.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>