PCI MSI: let drivers retry when not enough vectors

pci_enable_msix currently returns -EINVAL if you ask
for more vectors than supported by the device, which would
typically cause fallback to regular interrupts.

It's better to return the table size, making the driver retry
MSI-X with less vectors.

Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>