target/ppc: Fix non-maskable interrupt while halted

target/ppc: Fix non-maskable interrupt while halted

The ppc (pnv and spapr) NMI injection code does not go through the
asynchronous interrupt path and set a bit in env->pending_interrupts
and raise an interrupt request that the cpu_exec() loop can see.
Instead it injects the exception directly into registers.

This can lead to cpu_exec() missing that the thread has work to do,
if a NMI is injected while it was idle.

Fix this by clearing halted when injecting the interrupt. Probably
NMI injection should be reworked to use the interrupt request interface,
but this seems to work as a minimal fix.

Fixes: 3431648272d3 ("spapr: Add support for new NMI interface")
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>