Not all interrupt controllers have a working implementation of
message-signalled interrupts; in some cases, the guest may expect
MSI to work but it won't due to the buggy or lacking emulation.

In QEMU this is represented by the "msi_nonbroken" variable.  This
patch adds a new configuration symbol enabled whenever the binary
contains an interrupt controller that will set "msi_nonbroken".  We
can then use it to remove devices that cannot be possibly added
to the machine, because they require MSI.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>