vl: Switch qemu_uuid to QemuUUID

Update all qemu_uuid users as well, especially get rid of the duplicated
low level g_strdup_printf, sscanf and snprintf calls with QEMU UUID API.

Since qemu_uuid_parse is quite tangled with qemu_uuid, its switching to
QemuUUID is done here too to keep everything in sync and avoid code
churn.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-10-git-send-email-famz@redhat.com>
