ppc/xive: Synthesize interrupt from the saved IPB in the NVT

ppc/xive: Synthesize interrupt from the saved IPB in the NVT

When a vCPU is dispatched on a HW thread, its context is pushed in the
thread registers and it is activated by setting the VO bit in the CAM
line word2. The HW grabs the associated NVT, pulls the IPB bits and
merges them with the IPB of the new context. If interrupts were missed
while the vCPU was not dispatched, these are synthesized in this
sequence.

Signed-off-by: Cédric Le Goater <[email protected]>
Message-Id: <20191125065820[email protected]>
Signed-off-by: David Gibson <[email protected]>