elf-ops.h: fix int overflow in load_elf()

This patch fixes a possible integer overflow when we calculate
the total size of ELF segments loaded.

Reported-by: Coverity (CID 1405299)
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20190910124828.39794-1-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>