target/ppc/kvm: Replace variable length array in kvmppc_save_htab()

To be able to compile QEMU with -Wvla (to prevent potential security
issues), we need to get rid of the variable length array in the
kvmppc_save_htab() function. Replace it with a heap allocation instead.

Message-ID: <20240221162636.173136-2-thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>