KVM: PPC: Book3S HV: Use msgsnd for IPIs to other cores on POWER9

On POWER9, the msgsnd instruction is able to send interrupts to
other cores, as well as other threads on the local core.  Since
msgsnd is generally simpler and faster than sending an IPI via the
XICS, we use msgsnd for all IPIs sent by KVM on POWER9.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>