KVM: ppc: distinguish between interrupts and priorities

Although BOOKE_MAX_INTERRUPT has the right value, the meaning is not match.

Signed-off-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>