KVM: PPC: add missing MODULE_DESCRIPTION() macros

With ARCH=powerpc, make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in arch/powerpc/kvm/test-guest-state-buffer.o
WARNING: modpost: missing MODULE_DESCRIPTION() in arch/powerpc/kvm/kvm-pr.o
WARNING: modpost: missing MODULE_DESCRIPTION() in arch/powerpc/kvm/kvm-hv.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
[mpe: Change kvm-hv description to mention POWER8 & later]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240615-md-powerpc-arch-powerpc-kvm-v1-1-7478648b3100@quicinc.com