util: Alter return value of virReadFCHost and fix mem leak

util: Alter return value of virReadFCHost and fix mem leak

https://bugzilla.redhat.com/show_bug.cgi?id=1357416

Rather than return a 0 or -1 and the *result string, return just the result
string to the caller.  Alter all the callers to handle the different return.

As a side effect or result of this, it's much clearer that we cannot just
assign the returned string into the scsi_host wwnn, wwpn, and fabric_wwn
fields - rather we should fetch a temporary string, then as long as our
fetch was good, VIR_FREE what may have been there, and STEAL what we just got.
This fixes a memory leak in the virNodeDeviceCreateXML code path through
find_new_device and nodeDeviceLookupSCSIHostByWWN which will continually
call nodeDeviceSysfsGetSCSIHostCaps until the expected wwnn/wwpn is found
in the device object capabilities.

Signed-off-by: John Ferlan <jferlan@redhat.com>