util: Add UUID API

util: Add UUID API

A number of different places across the code base use CONFIG_UUID. Some
of them are soft dependency, some are not built if libuuid is not
available, some come with dummy fallback, some throws runtime error.

It is hard to maintain, and hard to reason for users.

Since UUID is a simple standard with only a small number of operations,
it is cleaner to have a central support in libqemuutil. This patch adds
qemu_uuid_* functions that all uuid users in the code base can
rely on. Except for qemu_uuid_generate which is new code, all other
functions are just copy from existing fallbacks from other files.

Note that qemu_uuid_parse is moved without updating the function
signature to use QemuUUID, to keep this patch simple.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-2-git-send-email-famz@redhat.com>