msi_init: change return value to 0 on success

msi_init: change return value to 0 on success

No caller use its return value as msi capability offset, also in order
to make its return behaviour consistent with msix_init().

cc: Michael S. Tsirkin <[email protected]>
cc: Paolo Bonzini <[email protected]>
cc: Hannes Reinecke <[email protected]>
cc: Markus Armbruster <[email protected]>
cc: Marcel Apfelbaum <[email protected]>

Acked-by: Hannes Reinecke <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Cao jin <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>