vdi: don't override libuuid symbols

It's poor symbol hygiene to provide a global symbols that collide with a
common library like libuuid.  If QEMU links against a shared library
that depends on uuid_generate() it can end up calling our stub version
of the function.

This exact scenario happened with GlusterFS libgfapi.so, which depends
on libglusterfs.so's uuid_generate().

Scope the uuid stubs for vdi.c only and avoid affecting other shared
objects.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
