util - add automated ID generation utility

util - add automated ID generation utility

Multiple sub-systems in QEMU may find it useful to generate IDs
for objects that a user may reference via QMP or HMP.  This patch
presents a standardized way to do it, so that automatic ID generation
follows the same rules.

This patch enforces the following rules when generating an ID:

1.) Guarantee no collisions with a user-specified ID
2.) Identify the sub-system the ID belongs to
3.) Guarantee of uniqueness
4.) Spoiling predictability, to avoid creating an assumption
    of object ordering and parsing (i.e., we don't want users to think
    they can guess the next ID based on prior behavior).

The scheme for this is as follows (no spaces):

                # subsys D RR
Reserved char --|    |   | |
Subsystem String ----|   | |
Unique number (64-bit) --| |
Two-digit random number ---|

For example, a generated node-name for the block sub-system may look
like this:

    #block076

The caller of id_generate() is responsible for freeing the generated
node name string with g_free().

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>