vfio-pci: Fix multifunction=on

When an assigned device is initialized it copies the device config
space into the emulated config space.  Unfortunately multifunction is
setup prior to the device initfn and gets clobbered.  We need to
restore it just like pci-assign does.

Cc: qemu-stable@nongnu.org
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
