KVM: PPC: Implement mtsr instruction emulation

The Book3S_32 specifications allows for two instructions to modify segment
registers: mtsrin and mtsr.

Most normal operating systems use mtsrin, because it allows to define which
segment it wants to change using a register. But since I was trying to run
an embedded guest, it turned out to be using mtsr with hardcoded values.

So let's also emulate mtsr. It's a valid instruction after all.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>