storage_file_probe: Refactor qcowXGetBackingStore into specific callbacks for qcow...

storage_file_probe: Refactor qcowXGetBackingStore into specific callbacks for qcow and qcow2

Change qcowXGetBackingStore to use the new function prototype which
fills virStorageSource directly. Convert the copying of the backing file
string from 'g_new0' + 'memcpy' to 'g_strndup' as we treat it as a
string.

Introduce qcowGetImageSpecific (as a wrapper for qcowXGetBackingStore)
and qcow2GetImageSpecific. The latter of the two will be used to collect
all the qcow2-specific code later on, but for now it just parses the
backing store and the format.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>