Generally, functions which return malloc'd strings should be typed
as 'char *', not 'const char *', to make it obvious that the caller
is responsible to free things.  free(const char *) fails to compile,
and although we have a cast embedded in VIR_FREE to work around poor
code that frees const char *, it's better to not rely on that hack.

* src/qemu/qemu_driver.c (qemuDiskPathToAlias): Change return type.
(qemuDomainBlockJobImpl): Update caller.