virtio-pci: alloc only resources actually used.

Move resource allocation from common code to legacy and modern code.
Only request resources actually used, i.e. bar0 in legacy mode and
the bar(s) specified by capabilities in modern mode.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

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