util/cutils: Rename qemu_strtosz() to qemu_strtosz_MiB()

With qemu_strtosz(), no suffix means mebibytes.  It's used rarely.
I'm going to add a similar function where no suffix means bytes.
Rename qemu_strtosz() to qemu_strtosz_MiB() to make the name
qemu_strtosz() available for the new function.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-16-git-send-email-armbru@redhat.com>