kvm: introduce kvm_arch_msi_data_to_gsi

On ARM the MSI data corresponds to the shared peripheral interrupt (SPI)
ID. This latter equals to the SPI index + 32. to retrieve the SPI index,
matching the gsi, an architecture specific function is introduced.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>