util: hash: Change type of hash table name/key to 'char'

util: hash: Change type of hash table name/key to 'char'

All users of virHashTable pass strings as the name/key of the entry.
Make this an official requirement by turning the variables to 'const
char *'.

For any other case it's better to use glib's GHashTable.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>