virito-pci-modern: rename map_capability() to vp_modern_map_capability()

To ease the split, map_capability() was renamed to
vp_modern_map_capability(). While at it, add the comments for the
arguments and switch to use virtio_pci_modern_device as the first
parameter.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-16-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>