list: Add helpers for listing node devices

list: Add helpers for listing node devices

src/conf/node_device_conf.h:
  * New macro VIR_CONNECT_LIST_NODE_DEVICES_FILTERS_CAP
  * Declare virNodeDeviceList

src/conf/node_device_conf.c:
  * New helpers virNodeDeviceCapMatch, virNodeDeviceMatch.
    virNodeDeviceCapMatch looks up the list of all the caps the device
    support, to see if the device support the cap type.
  * Implement virNodeDeviceList

src/libvirt_private.syms:
  * Export virNodeDeviceList
  * Export virNodeDevCapTypeFromString