target-i386: Add API to get note's size

We should know where the note and memory is stored before writing
them to vmcore. If we know this, we can avoid using lseek() when
creating vmcore.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>