correctly check ppr priority during interrupt injection]

TPR blocks all interrupts in a priority class, so simple "less or
equal" check is not enough.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
