powerpc/kvm: Fix kvmppc_core_pending_dec

The vcpu->arch.pending_exceptions field is a bitfield indexed by
interrupt priority number as returned by kvmppc_book3s_vec2irqprio.
However, kvmppc_core_pending_dec was using an interrupt vector shifted
by 7 as the bit index.  Fix it to use the irqprio value for the
decrementer interrupt instead.  This problem was found by code
inspection.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>