util: Add one helper virReadFCHost to read the value of fc_host entry

util: Add one helper virReadFCHost to read the value of fc_host entry

"open_wwn_file" in node_device_linux_sysfs.c is redundant, on one
hand it duplicates work of virFileReadAll, on the other hand, it's
waste to use a function for it, as there is no other users of it.
So I don't see why the file opening work cannot be done in
"read_wwn_linux".

"read_wwn_linux" can be abstracted as an util function. As what all
it does is to read the sysfs entry.

So this patch removes "open_wwn_file", and abstract "read_wwn_linux"
as an util function "virReadFCHost" (a more general name, because
after changes, it can read each of the fc_host entry now).

* src/util/virutil.h: (Declare virReadFCHost)
* src/util/virutil.c: (Implement virReadFCHost)
* src/node_device/node_device_linux_sysfs.c: (Remove open_wwn_file,
  and read_wwn_linux)
src/node_device/node_device_driver.h: (Remove the declaration of
  read_wwn_linux, and the related macros)
src/libvirt_private.syms: (Export virReadFCHost)