virStorageVol: avoid PATH_MAX-sized array

virStorageVol: avoid PATH_MAX-sized array

POSIX allows implementations where PATH_MAX is undefined, leading
to compilation error.  Not to mention that even if it is defined,
it is often wasteful in relation to the amount of data being stored.

All clients of vol->key were audited, and found not to care about
whether key is static or dynamic, except for these offenders:

* src/datatypes.h (struct _virStorageVol): Manage key dynamically.
* src/datatypes.c (virReleaseStorageVol): Free key.
(virGetStorageVol): Copy key.