spapr_pci: Fix number of returned vectors in ibm, change-msi

Current guest kernels try allocating as many vectors as the quota is.
For example, in the case of virtio-net (which has just 3 vectors)
the guest requests 4 vectors (that is the quota in the test) and
the existing ibm,change-msi handler returns 4. But before it returns,
it calls msix_set_message() in a loop and corrupts memory behind
the end of msix_table.

This limits the number of vectors returned by ibm,change-msi to
the maximum supported by the actual device.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: qemu-stable@nongnu.org
[agraf: squash in bugfix from aik]
Signed-off-by: Alexander Graf <agraf@suse.de>
