target/ppc: Support directed privileged doorbell interrupt (SDOOR)

target/ppc: Support directed privileged doorbell interrupt (SDOOR)

BookS msgsndp instruction to self or DPDES register can cause SDOOR
interrupts which crash QEMU with exception not implemented.

Linux does not use msgsndp in SMT1, and KVM only uses DPDES to cause
doorbells when emulating a SMT guest (which is not the default), so
this has gone unnoticed.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20230530130526.372701-1-npiggin@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>