hw/nvme: Use pcie_sriov_num_vfs()

hw/nvme: Use pcie_sriov_num_vfs()

nvme_sriov_pre_write_ctrl() used to directly inspect SR-IOV
configurations to know the number of VFs being disabled due to SR-IOV
configuration writes, but the logic was flawed and resulted in
out-of-bound memory access.

It assumed PCI_SRIOV_NUM_VF always has the number of currently enabled
VFs, but it actually doesn't in the following cases:
- PCI_SRIOV_NUM_VF has been set but PCI_SRIOV_CTRL_VFE has never been.
- PCI_SRIOV_NUM_VF was written after PCI_SRIOV_CTRL_VFE was set.
- VFs were only partially enabled because of realization failure.

It is a responsibility of pcie_sriov to interpret SR-IOV configurations
and pcie_sriov does it correctly, so use pcie_sriov_num_vfs(), which it
provides, to get the number of enabled VFs before and after SR-IOV
configuration writes.

Cc: qemu-stable@nongnu.org
Fixes: CVE-2024-26328
Fixes: 11871f53ef8e ("hw/nvme: Add support for the Virtualization Management command")
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20240228-reuse-v8-1-282660281e60@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>