vdpa: handle irq bypass register failure case

LKP considered variable 'ret' in vhost_vdpa_setup_vq_irq() as
a unused variable, so suggest we remove it. Actually it stores
return value of irq_bypass_register_producer(), but we did not
check it, we should handle the failure case.

This commit will print a message if irq bypass register producer
fail, in this case, vqs still remain functional.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20201023104046.404794-1-lingshan.zhu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>