hw/pci/pci-stub: Add msi_enabled() and msi_notify() to the pci stubs

hw/pci/pci-stub: Add msi_enabled() and msi_notify() to the pci stubs

Some machines have an AHCI adapter, but no PCI. To be able to
compile hw/ide/ahci.c without CONFIG_PCI, we still need the two
functions msi_enabled() and msi_notify() for linking.
This is required for the new Kconfig-like build system, if a user
wants to compile a QEMU binary with just one machine that has AHCI,
but no PCI, like the ARM "cubieboard" for example.

Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>