Finding virPCIE* code is more intuitive if located in virpci.h
instead of node_device_conf.h.

* src/conf/node_device_conf.h (virPCIELinkSpeed, virPCIELink)
(virPCIEDeviceInfo): Move...
* src/util/virpci.h: ...here.
* src/conf/node_device_conf.c (virPCIELinkSpeed): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>