pcie: Introduce pcie_sriov_num_vfs

pcie: Introduce pcie_sriov_num_vfs

igb can use this function to change its behavior depending on the
number of virtual functions currently enabled.

Signed-off-by: Gal Hammer <gal.hammer@sap.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit 31180dbdca2859ae9841939f85158908453ea01d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: needed for v8.2.0-2290-g91bb64a8d2
 "hw/nvme: Use pcie_sriov_num_vfs()" (CVE-2024-26328))