util: Emancipate id_wellformed() from QemuOpts

IDs have long spread beyond QemuOpts: not everything with an ID
necessarily goes through QemuOpts.  Commit 9aebf3b is about such a
case: block layer names are meant to be well-formed IDs, but some of
them don't go through QemuOpts, and thus weren't checked.  The commit
fixed that the straightforward way: rename the internal QemuOpts
helper id_wellformed() to qemu_opts_id_wellformed() and give it
external linkage.

Instead of using it directly in block.c, the commit adds wrapper
bdrv_is_valid_name(), probably to hide the connection to QemuOpts.

Go one logical step further: emancipate IDs from QemuOpts.  Rename the
function back to id_wellformed(), and put it in another file.  While
there, clean up its value to bool.  Peel off the bdrv_is_valid_name()
wrapper.

[Replaced stray return 0 with return false to match bool returns used
elsewhere in id_wellformed().
--Stefan]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

include/qemu-common.h

@@ -190,6 +190,9 @@ int64_t strtosz_suffix_unit(const char *nptr, char **end,
 /* used to print char* safely */
 #define STR_OR_NULL(str) ((str) ? (str) : "null")
 
+/* id.c */
+bool id_wellformed(const char *id);
+
 /* path.c */
 void init_paths(const char *prefix);
 const char *path(const char *pathname);