fix a ambiguous output of the command:'virsh vol-create-as'

I created a storage volume(eg: test) from a storage pool(eg:vg10) using
the following command:"virsh vol-create-as --pool vg10 --name test --capacity 300M."
When I re-executed the above command, the output was as the following:
"error: Failed to create vol test
 error: Storage volume not found: storage vol 'test' already exists"

I think the output "Storage volume not found" is not appropriate. Because in fact storage
vol test has been found at this time. And then I think virErrorNumber should includes
VIR_ERR_STORAGE_EXIST which can also be used elsewhere. So I make this patch. The result
is as following:
"error: Failed to create vol test
 error: storage volume 'test' exists already"

include/libvirt/virterror.h

@@ -296,6 +296,7 @@ typedef enum {
     VIR_ERR_ACCESS_DENIED = 88,         /* operation on the object/resource
                                            was denied */
     VIR_ERR_DBUS_SERVICE = 89,          /* error from a dbus service */
+    VIR_ERR_STORAGE_VOL_EXISTS = 90,         /* the storage vol already exists */
 } virErrorNumber;
 
 /**